Can you turbo an older motor ie with 150 200,000 mi or will it detenate? Just bought an older 1991 chev with the 6.2 and was wondering. Also I have not picked it up yet but what will a 1991 c3500 crew cab with a 6.2 and the 700r4 tow max wieght reasonably.

Can't speak for your vehicle but I just bought an '84 with an aftermarket Banks turbo that was installed in '97. The owner gave me a pile of receipts and it looks like the turbo was installed at around 97K, the vehicle is near 200K now and appears to be running well.

Yes, you can put a turbo on a higher mileage engine, but some preventative maintenance should be done to prevent problems. It would be a good time to roll in some new bearing inserts and replace the head gaskets and head bolts and possibly a fresh timing chain set. You could just put the turbo on and hope for the best, but with that many miles, you are taking a gamble.

Yes install new connecting rod and main bearings.
So far it is the best $120 I have put into my engine.
It gained me 20psi of oil pressure when hot from idle to the max.

Don't forget the cam bearings. I've seen a couple of "spun" cam bearings (rear bearing) on the early 6.2 blocks. It turns a good block into a big paper weight. Worn cam bearings will cause low oil pressure as well. Even more so than crank bearings.

Yea the oil goes to the cam bearings first in the 6.2L
I don't think you can change the cam bearings with out compleatly tearing down the engine.
Can you?