66 Valient, Exhaust Manifold Options

What are your expectations in spending the time and trouble for this? How hard are you running the 318? Doubt you won't seen much (any?) difference in normal (under 3500 RPM) between a stock early A and any other factory manifold. There is just not enough exhaust gas to even notice the restrictions in the early manifold at normal driving RPM. Headers are another conversation though. If the tubes are long enough it can make a noticeable difference in normal driving RPM. I run the stock early A on Edelbrock heads on my 360 because I never get it above 3500 RPM.