[hemi43]

Scrap the big block.
Numbers matching on these trucks makes no difference on value. You wont regret installing a 5.3 with a 4L60E. If you go the 6.2 route, I would probably want the 4L80E.
If you're handy, the swap is not expensive at all. Best thing to do is find a fairly low mileage 2002 Chev/GMC truck in running condition.
The day I bought my Blazer project, I also bought a rotted 2003 GMC with around 160k miles on it for $500. All I did was pull the rocker cover off to make sure the engine wasn't full of crap, and replaced all the gaskets. These engines when properly maintained should be good for 400+K miles. The transmission will have to be rebuilt anyways because you will need to install the Advanced Adapters output shaft.
If you do the work yourself, it shouldn't cost anymore than around 3K for the swap.
I mentioned trying to find a 2002, and that's because its the last year of drive by cable.
You will however need to change the gearing in the diffs which is an added cost, but again, fairly reasonable if you do it yourself.
IMHO, any engine more than 25 years old should be at the end of a chain to be used as a boat anchor.
I did my swap 4-5 years ago, have put over 20K miles on it, and get well over 20 MPG. It always start, tons of power, and doesn't stink going down the road. My only regret is using a 2003 with drive by wire. Works great, but has a lousy pedal feel which can't be tuned out on a factory computer. If I was to do the swap again, I would definitely use an aftermarket computer to control both the engine and transmission.