This is good input, so here is the rest of the "plan". Late '70's Dodge P/U, shortbed, w/ hotrod /6, 5-sp Tremec, 8 3/4 rear. Stock frame but independent front suspension and coil-over rear. Keep it as much Mopar as possible. OK- now you can go crazy.
How do plan on using the truck? Sounds like you have a decent budget. If planning on using it as a truck (to haul things) Then I would consider swaping to a V-8.
I Have a 1984 D-100, slant six, super six conversion, A-833OD trans, 3.91SG 8 3/4 rear, A/C
Haven't had for very long and still need to make improvements. Get upper teens, running empty, and got 13-14 mpg towing my race car to kentucky and back from Florida. It did OK, but could use a little more power for long grades, or steep hills. Biggest problem I have is the clutch (not uncommon with a slant, as they have a small clutch) . Really have to baby it. I don't know it's actual condition, it could be just about worn out, or possible oil on it as the rear main is leaking. As soon as I have time, I will pull the engine to reseal, and install new clutch.
PS: I also have a 88 D-150 fully loaded, with a 360 4bbl/727. It pulls just fine, but gets about 9 mpg towing, and 10 empty.
Choose your poison.
