The schematic for a 917.256611 doesn't match your description of "turn the key to light"....

It shows a separate AC lighting circuit controlled by its own switch.

IF it actually is that model, the only 2 things that are fed by the key switch ON circuit are the carb fuel solenoid (BLUE) and the PTO switch (RED).

I'm not sure if it will match the schematic 100%, as it appears that the original Kholer engine has been replaced by an 18HP Briggs and Stratton. The ignition has three positions: off, light/run, and start. The fuse is blowing when turning it to crank the engine.

Sure you don't have a digit wrong?
The number you posted has a SEPARATE light switch, not part of the key switch.

If the fuse blows ONLY when you turn the key to the START position, then you have probably pinched the white wire that makes up the solenoid control wire somewhere in your repair process. This white wire leaves the key switch and goes first to the PTO switch, then it goes to the clutch/brake switch. From there it goes to the starter solenoid. You need to go back and retrace all your steps taken when you had everything apart repairing the steering to see if a wire is caught between some other parts/screws/nuts/bolts.

"the fuse blew every time I turned the key to light."
"The fuse is blowing when turning it to crank the engine."

When you make up your mind, let us know.
Troubleshooting is different for each!

.@mownie, thank you for your input. I will double check that white wire first thing tomorrow morning and let you know what I find.

.@ bill_kapaun: I have made up my mind. If you had read my original post, "I have replaced the starter, the starter relay, the fuse housing, and the ignition switch. Now I can turn the key to the light position without the fuse blowing, but as soon as I go to start the mower, it blows without the engine or starter doing anything." And yes, I'm sure I don't have a digit wrong. There is a separate light switch for the headlights, but it won't work unless the mower is running or the key is turned to the "light" position.

The schematic in the OM (link below page 31/60) doesn't show it.
The key switch in the schematic has only 5 terminals.
To have a light position as part of the key switch requires more than that.

Since the headlights operate off the AC lighting circuit from the engine, a light position on the key switch would be totally redundant with a separate light switch.

I think there's been some "surgery" to the wiring harness??

Here is a link that might be useful: Owners manual

There has been some surgery. The ignition switch had 6 terminals, but the replacement I purchased for the mower only has 5. There has also been wiring work done when the engine was replaced with an 18HP Briggs and Stratton, which is why I have had difficulty following the wiring schematic.

Not all ignition switches are wired the same internally, although they may look identical externally.

Did you use the correct part# switch?

In my many repairs of wiring, ignition switches, and et cetera, i have found this to be true, regarding the ignition switch:
Buy the correct one for that machine! Take the tractor numbers, engine numbers, and make/name of machine to a lawn tractor parts place, and get the correct switch!
Or, check the Sears/Craftsman web-site, where you can access the parts section, and find it there. They will send the parts to your home, but it is pricey! I am lucky---i have a very good parts place nearby. I give them all of the numbers, etc., and they usually have the part, but if not, they can have the correct part in a couple days.

The old switch is not the original switch on the mower. The factory switch had 5 terminals, and the switch that was on it when I purchased it had 6. Whomever put the new switch in, all they did was cut into a ground and splice it to the 6th terminal. I have reversed that and put in a factory replacement switch with 5 terminals.

I have removed the entire tractor wiring harness from the tractor, inspected every inch of wire, made new connections where needed, and covered exposed wires. I have checked both major grounds, sanded down any rust for a clean connection, and re-posted the grounds. One by one I have removed different parts of the circuit including the starter, the ammeter, the PTO switch, the PTO harness running to the main pulley beneath, and the clutch switch. The fuse still blows each time I turn the key to the third position.

I do have a multimeter, and I have tested the battery, as well as the old and new ignition switches. I'm completely baffled. The only connectors I did not replace are the harnesses for each connection, and the plugs for the black and white wires running to the small posts of the starter relay.

There also appears to have been other wiring work done on the tractor prior to my purchasing it. I believe two wires from the PTO switch were removed and spliced together, but it ran just fine with that setup previously, so I do not think that would be causing the issue.

OK, do this: For these tests, leave the key switch off at all times. Take the small white wire loose from the solenoid. Next, remove the white wire from the key switch. Using an Ohmmeter, test from the white wire at the solenoid to a good chassis ground. Ideally, the circuit should be OPEN (no continuity). If you detect continuity at this point the white wire is shorted to a ground somewhere (maybe inside the harness?) If no continuity, good. Keep going. Next, check the white wire (both ends still disconnected) by connecting the Ohmmeter leads to both ends of the white wire while the brake pedal is down, PTO switch OFF. You should read continuity at this time. If you read continuity at this time.......the problem is not in the white wire circuit at all and this result likely means that the white wire has been connected to the wrong terminal on the switch. Worst yet.......this also means that the key switch terminal the white wire SHOULD BE connected to, may have a ground wire connected to it. This means that when you turn the key to the third position you are not sending current to the white wire but instead to a grounded wire......fuse go kerpow...flash....sparkle in the dark.

Mownie gave you good advice-

Something that sticks out at however is-
"...and the plugs for the black and white wires running to the small posts of the starter relay."

The schematic shows ONE small terminal (White from key switch S) with the ground basically accomplished by the solenoid being physically attached to the sheet metal/frame.

I wonder if this solenoid (probably a replacement?) still grounds through the "body" and the 2 wires are switched?
This would cause a dead short to ground in the crank position.
Just throwing that out there-

One could test with the ohm meter-
Check resistance between the 2 small terminals. It should be fairly low, but "readable". Note the reading.
Check each terminal to ground.
One (white wire) should have the same reading and the other (black wire) should have ZERO resistance.