Well now that we’ve left fuel tanks and moved on to engines. (That part is easy on these trucks. Can buy everything needed at reasonable prices for whatever OE style setup you want. No reason to try to over engineer in this area. Especially since you have a flatbed so fuel filler is easy in about any location.)
If you’re hoping to recoup the cost of a LS swap in fuel savings, ain’t gonna happen. Not even close. Especially if you’re keeping a non OD trans (more $ and time to swap that).
But turning a 40 + year old truck into anything that resembles an efficient comfortable high miles daily driver is not going to be cheap. It can be reasonable if you start with the right vehicle, like
@SirRobyn0, you can get there. But HP costs $. Period. And he would have spent a lot more by now if he was pushing more power than a stock 80s small block produces. And he fell into a reliable truck with OD for cheap.
I can tell you an early 5.3/4L60 4wd Tahoe is 13-14mpg all the time. Main duty is 50-60 mi commutes mostly highway. Sometimes stop n go sometimes 85mph. Straight 80mph highway = 13 mpg. About the same weight as an old square. But it probably has 3.42 gears.
Speed makes a huge difference though. What speed is your mpg at?
Not 80-90mph presumably.