6-71 vs. 8-71

Less than 400 cubes, I'd go with a 6-71. I ran one on a 68 426 Hemi for a number of years on the street. It made plenty of boost, and I ran a bit of overdrive on it.
A 318 with a 6-71 is a great match. One thing though. You probably already know to build the engine with forged everything. Don't waste you time, or risk the engine by trying to save a buck with cast parts.