[JJH Jimmy]

I think trying to get 18 mpg in a squarebody going to be tough but there are my suggestions.

Buy a crate 350 with roller cam for reliability.

Find a good used quadrajet carb and have it rebuilt by a reputable builder.

700R4 trans since computers aren’t your thing and have it rebuilt with all the upgrades to make it last. You should be able to pick up a good core for a couple hundred dollars. (I personally hate 700r4’s but they can be built to last). You will never have good mpg with a 3 speed auto.

Or

If you don’t mind a manual nv4500.

Np208 transfer case because they are plentiful/cheap and are fairly reliable.

I think it will be hard to have an engine rebuilt cheaper than buying a crate engine unless you do the work yourself. Buy a crate engine from a reputable company that offers a warranty. I have a blueprint in my K20 and have been really pleased with it.

Put 3.73 gears in it.

With all that being said you might get 14 or 15 mpg.

That’s just my thoughts! Sounds like a fun project.