I can get a ls1 rear end with disc for 100. Should i get it? Itll bolt right up right or do i need to change anything? Will the third gen wheels stick out far?

might have to change porpotioning valve, but it'll be direct swap. you'll have about 1.5" per side more length and you'll have a 7.5/7.62 ten bolt, most likely like you already have.

can i take the ring and pinion outa my rearend and put in the ls1?

If it is from a 28 spline 10 bolt then yes.

your car is a 91 z-28, so yes. just replace the crush sleeve and reset your pinion depth and backlash.

it has the ebrake cables can i use them or no?

if this came out of an ls1 car, i dunno. haven't seen them to be able to tell you. if it's out of an LT one car, you use your old ones, but not the new ones, because the ends are different.