340 manifolds vs Summit Headers

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?

What youll lose top end youll pick up bottom end. Tested on a stock motor, headers did jack over 340 logs. Both slightly better than Lo-po logs. I would do it in a heartbeat (and I have had them, as well as cheapo headers and expensive fenderwells too) Warmer motor demands headers and GOOD headers (think flange, heavy walled drop in style, TTI, Dougs, top shelf stuff) would be the only think I would consider after 340 logs (the early Dakota's are very good too, although not as "accepted"). Do it.

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