[tucsonjwt]

I think the original question concerned gas mileage, at least in part.
I have owned many 5.3 and 4.8 new GM trucks and SUVs. In those new vehicles, with very careful driving, I got 17 mpg city and 23 mpg highway, whether it was 4.8 or 5.3. Rear axle ratio is important -3.23 best for mpg, 3.73 worst. However, my latest new vehicle was a 2010 GMC Sierra Ext. Cab with 4.8 and 3.23, and that got 15.5 city and 19 highway. If I did more conventional (less stop and go) driving I think I would have gotten 17/23 on that vehicle also. I am very anal about keeping track of mpg, and drive like your grandma (not the one from Pasadena.)
Some practical considerations would rule out the LS swap for me. In AZ you are not allowed to swap engines according to emissions rules, even if the replacement engine produces less emissions. You must have factory original engine and all factory smog components in place. Some say they get through emissions with modified/newer engines, but I always get super inspected when I go to emissions and would never pass - plus I think there are technically fines for this kind of mod.
As stated above, mpg would not improve much for a swap, assuming you are comparing the engines in a lightweight 1/2 ton truck. Most old squares get 10-12 city, 13-15 highway - but I think you can do better with a V6 and open rear end and overdrive transmission - especially highway.
I only drive my truck about 3000 miles per year, but it is a 454/SM465 3/4 ton beast with a dump bed conversion, so it is suited for my limited purpose as is, even if it is only 8-11 mpg city - which is the only place I drive it. It sits idling at stop lights more the it drives down the road.
As most forum members posting here know, the greatest cost in owning these old squares is all of the non-drive train components - over time. All of the electrical/mechanical/interior trim parts add up over time. While this is ultimately true for any vehicle, it really makes more economic sense to buy a newer (or new) vehicle and limit your other expenses. Modern trucks with LS engines in decent shape go for $7-10K and you won't have to put much money in them if you get a good one.