Do all the blades turn free on their individual spindles (with the blade secondary belt removed)?
When you engage the PTO, does the engine sound like it has a load on it? or does it just stop running as though you had turned off the ignition?
When you engage the PTO, can you actually hear and feel the deck engage? or does the engine just die outright as soon as the PTO switch is activated?
Did the problem show up after you did the work on the deck (R&R belts, blades, pulleys) or was that work done attempting to solve the problem of the engine dying?
If the engine DOES NOT seem to be under a load, but simply dies like the key was turned off when you engage the PTO, and all the spindles and pulleys on the deck are free to move, I doubt the issue has anything to do with the deck assembly itself.
If the engine just cuts off and stops when the PTO switch is activated and nothing tries to move regarding the deck and blades, I would suspect a defective PTO switch (PTO switch would have internal short to ground on its magneto circuit).
Looking at the wiring diagram for your tractor, I see that if the seat switch were defective, the engine would die every time you let up on the clutch/brake pedal, but you don't say it does this.
If the engine does act like a load has been applied to it, and you don't smell or see smoke from a belt, or hear belt squeal, I would suspect the carburetor needs a good cleaning (carb disassembly required, not just spray into carb throat).
If you will carefully answer all the questions I put up we can better determine exactly what you have going on.