full-size rearend on an s-10

IM converting my 2.8l v6 to a350 v8 in my 87 -10. i know im goingto need a different rearend, so can i just pull one of a 70's-late80's fullsize andhaveit narrowed?

Yes.
But your seriously better off just beefing up the stock rear end.
My second choice would be a Ford 9"
They are cheaper to build up than a Chevy, and stronger

how would ibeef up the old one?

the old one SHOULD hold up... One of my very good friends has a motor wiht 515 hp at the flywheel
in the stock 1994 rearend. It holds up great. and he even has slicks on it (though i wouldn't say you will have this luck too). Just run it and see how it goes..

If you rip out the gears buy new ones and a locker while your at it.

I spoke to my mechanic / drag racer friend today, and he sayes that the most that you'll need is a Auburn Locker.
The way that he made it sound they make a complete drop in unit for your rear end.
You might also want to run aftermarket axles.

He also sayes that unless your running Big power, a stock rear end should easily hold up