As for the better bottom end. What rearend does your truck have? For bottom end (motorwise) you could change the cam, maybe 260* Adv. Duration 427" G. lift Or 266*-440". The carb sounds good. Don,t get to big unless you run WFO. If you want a man. tran. The chevy truck 4-speed with granny low (1st gear) is just a junkyard away. Theey grow on trees. It would give you about the same highway as your 3sp auto. Also a 5-speed chevy man. from truck in junkyard or classified. Check into gearvender on internet search. They bolt to the rear of your tranny and give you a high and low in every gear that you already have. I'm putting one in my crewcab (83 1-ton 3+3) after this winter Boiler season
I had a 1973 1-ton with 350, truck 4-speed. The 350 was stock, like 170 some horse but the low first gear made it a haws. It's a workin'mans tranny though, like shifting a tractor. You can pull off from a stoplight in 2nd gear if you want though, 1st gear is real short unless you are pulling. I've driven the 5-speed before and seen a friend tear one up. they don't seem as tough. With that year truck, you can do pretty much what you feel like. Research pro's and con's on diffrent setups. God bless the internet. Also check on the front of your heads. See if there is a Mark that looks like the two humps (^^) on a camels back. These heads have bigger valves than the others. Worth more ect. PS, don't know what you've done allready but run through everything an replace poor wornout stuff; plugs, wires, points, filters oil-tranny, oil, air filter, all the stuff mister 17,000 said he did. I'm struggling with my brakes and steering because the last owner just put gas in the fu$$er.