I just took my headers off my 72 monte carlo. It has roughly the same 402 as you scotty. I spent 3 hours on my manifolds by grinding out the air fitting humps inside the port and smoothing out the turns. Knocked off the casting flash as well. I wanted the engine to look stock again as i put the car in shows. I did not notice any difference in performance but the exhaust sounded deeper( better in my opinion). The car ran a 14.38 with headers and 14.44 with the manifolds. I'll take the manifolds any day unless you are building real hp, Which you are not because it's a 402. Now i can pull the oil filter off without pulling the driver side header off.