The RARE large manifolds are available in roundport. It's the D ports that are almost available. RARE has what appears to be some pretty firm production dates.
From everything I've read, the 3" (Super Long Branch) manifolds will only net you an improvement if you're running a monster motor as Q referred to. For that configuration you might net 20-30 HP over standard RA manifolds (based on extrapolation from data I've seen). With a stock or mild 455 or smaller, you probably won't net any HP increase over standard RA manifolds.
