Take/send your heads to get ported and polished and at least bigger exhaust valves by a shop that knows and loves these motors. Get a good RV cam, mechanical secondary carburetor about the 625 cfm to 650 cfm range anything bigger is a waste of money for a stock to mild built 460 as the exhaust port in the stock heads is too restrictive for much bigger. I would not swap in a smaller gas engine if you live by mountains or plan to be pulling passes a lot, not to pick on flat landers but a lot of them don't realize big blocks reigned in the mountains (before the dawn of turbo diesels) for a very good reason. I have seen many small block engines meet their doom on the passes here in Colorado.
I had an 87 F350 crew cab long bed 460 5 speed with 4.10's that was built to the above that would get 14 mpg on the highway empty and 8-12 depending on how much I towed.
You have quads or dirt bikes for the tight stuff. I would use the truck as a base camp and explore out from there.
A cummins swap would also be sweet, but as said doing it just for economy is not going to pay out very quickly if ever.
1993 F350 Crew Cab Long Bed 7.3 IDI 5 speed 4x4. Currently stock except for the 255/85R16's.
1983 GMC 6.2L Suburban with sm465/np205 with 1 ton axles, crossover steering, 52" front springs, and rear shackle flip. Has 285/70r17 STT's on H2 rims.
1996 GMC Suburban K2500 6.5TD 4L80E
1965 2WD F100 7.5L with C6.
1967 mustang coupe project on hold waiting for a garage or shop.
1979 CX500 putt arounder.