No matter what you do it will be very hard to get the base level on that old pad. If that pad was lifted to make it unlevel whats to say it won't lift again someway or crack and lift. Then you'd be struck with it off kilter shed. I'd either get rid of the pad or find another place to put your new shed

I concur with the above recommendation - remove the slab, or relocate the new shed. It appears the old concrete slab may been poured on-grade, with disregard for the minimum frost depth for your area.

Assuming your minimum winter temps do drop below 32ðF (0ðC), it's best to place the new shed on a level, compacted gravel base to provide good drainage, hence avoiding frost heave. There are right and wrong ways of constructing a gravel foundation as well; follow the shed company's recommendation.

My custom 8 x 16 shed has pressure-treated 2x6 joists 16" oc spanning 8 ft (doubled at rims) and is being built over six poured concrete piers, 8 ft on-center with footing bottoms 42" below finished grade. If you were to take that approach, your 8 x 16 tuff-shed, with 6" galv steel joists @ 2'-0" oc, may require simiar spacing of conc. piers (or piles).

This foundation design is relatively inexpensive, requires a bit more effort to construct, but is the best method for minimizing risks of future settling or frost heaving.

Here's what I did for my 10x16 shed I got last year:

{{gwi:27479}}
{{gwi:296170}}

2-1/2 pallets of stacked stone, back-filled with processed gravel.

I made the "foundation" about 1 foot wider all around, and bought generic pavers as a cap.

It took about 2 days to do.

Nice shed foundation GardenGuy . . . but since we're playing show me your best foundation, instead of helping the OP, here's the foundation built for one of my houses . . . took the masons 3 months to lay hundreds of tons of stone, with custom cut 2" bluestone caps ;-) :

{{gwi:296171}}

{{gwi:296172}}

Thanks for the suggestions, everyone! I'll let you know how the new foundation works out (we're leaning toward getting rid of the concrete and putting down compact gravel as a base. Thanks again.