[D13]

How much money do you want to spend?
1) bottom end grunt for pulling?
2) Quick revving for performance?
3) Gobs of HP up top?
4) max fuel economy?

First buy the Second edition of Santucci's book and join Inliners.

If you are looking for low speed driveability out of the generation of inlines in our trucks, get rid of the integral head, put on a head with 1/8 intakes / 1.6 exhaust, lump ports, 307 (higher compression) or forged pistons, and the 'marine' cam that makes a little more grunt than stock, Langdon's iron headers, Offy intake and 390 Holley.
More torque with less economy, use a 292 as the base (they aren't efficient but they really pull).
Good economy and some hard pull at upper RPM, use the turbo exhaust mainfold with a small T4 sized to hit at 2500rpm (above cruising RPM) and switch to TBI (better fuel mixing).
Blowers are $$$ and the block is the limiter (fails ~700HP).

Remeber these motors are antiques. If you want it to drive like a modern vehicle you will need to swap the motors, either LS or 4200 Atlas, or spend a whole wad of cash.