I bet your local machine shop can have you a 350 turned around for $500-700 within a week
Nothing wrong with what you're thinking with the cheap, reliable. I wonder if you could scrounge up a cheap TBI Suburban or truck and just swap that over? You'd get the overdrive of a 700r4 (that'll get you at least 3-4 mpg) and pretty similar reliability to a carbed 350. Might get lucky and find one with a rebuilt trans or engine?
You have done the trans. I might just do a fresh build on the 350 and roll with it how it is. Sounds like you're up for that
(just an observation on the body of the truck....I'm not sure if you're planning a full restore or if you're just going to run it around as is with a reliable drivetrain, but if you do restore the body, I'd be surprised if you're not putting cab corners, rockers and floors in the truck with the way the door looks----personally, I'd make it reliable and drive it as is and leave the body) The engine could just keep going for a long time? Who knows?