Yeah, it wouldn't be to save just gas. The engine seems pretty strong. In the event it goes, I figure instead of a rebuild, it would be nice to replace, to get more power, and maybe some better gas mileage to boot.Engine technology helps a little. According to fueleconomy.gov, a '98 C1500 Suburban picks up 1 mpg over a '91 R1500 Suburban. I wouldn't be surprised to see you pick up 1 mpg if you installed a newer 350. Of course, what is it about the newer 350 that gives it the extra mpg: the better flowing heads, the engine management system, or ???
I expect it would take years to recover the cost, unless you had some other reason for replacing the engine, such as the existing engine needs rebuilt anyway.