Just on a 68 318 if I swapped from headers to 340 manifolds if I would loose any significant power? The 318 is stock other than a Duration 262/272, Lift .420/.442 Summit Cam and 1406 (600 cfm) Edelbrock carburetor and performer intake with electronic ignition.

The car gets warm with the headers close to the floor pan/underhood and I have access to a set of these manifolds which I think may be 68-69 vintage and potentially free of cost if not dirt cheap from a family member if they don't sell them before. Also will these fit a 68 Coronet small block with ease? I imagine they would but want to be sure before I jump a them?

Opinions?