The venerable Chevy 350 is lighter than the straight 6 and has a LOT more torque. It's actually a very good upgrade. Cheaper to build and parts are plentiful.
I don't see why it's a "big mistake" to keep a manual transmission in a truck. A common Muncie M20 wide ratio or M21 close ratio will handle about 450 lb-ft of torque, gives you a 4-speed, and bolts up nicely to a 350 with an equally common bellhousing. If you want an overdrive, Gear Vendors makes one for the Muncie. Really pretty basic, a lot more powerful, and a lot smoother.
I can see exactly why the hotrodders like the 350 so much. Plus it's better for the purists...more stove bolts motors for sale on e-bay.
