Power loss. 318 Vs 340 manifolds

Since you are running 302 heads with 318 ports I'd suggest the 318 manifolds. The ports on a 360 and 340 are quite a bit bigger. You will have more sealing area, and with Dual exhaust it should still run great. I had a 64 Barracuda with a 273 Commando with 72 340 heads and cam, a 71 340 manifold and TQ. I used the stock exhaust manifolds, Match Ported them and had custom dual exhaust installed. The car ran great and was trouble free.