66 Dart 360 Swap Questions

Some porting and smoothing can help a bit. Extrude honing can do a bit more, but, it's expensive and only helps just so much. You get the casting thinner and then you start having crack problems. Anything bigger than a 318 and I'd recommend headers if you plan on getting high RPM's. If it's just a street car and never sees 5k, the stock manifolds will work fine for flow. It's the ports sizes that can be a problem if you use 340/360 heads. The gasket tends to blow out easily, and depending on which casting you use, #7 exhaust port area can interfere with the manifold casting. Either the head needs to be ground on to remove extra metal (a delicate and risky job), or an extra spacer made (like a header flange) to bring the manifold out a ways to get clearance. If not, you try bolting the manifold down and it will break.