340 horsepower rating

I'm just finishing my 340 4 speed 68 formula s fastback resto. This car is a replacement for the "new" 68 formula s I had in high school in 1969.

My original barracuda had fenderwell hooker headers, 3.91 posi, 750 DP holley, and an edelebroc alluminum "high rise" manifold. It was a consistent mid to high 12 second car with open headers and slicks. The barracuda I'm building now has the same components except for "under chassis" hooker headers. These cars are just worth too much money now to take a reciprocating saw to the fenderwells.

Do any of you racer types know if a 340 produces less horsepower with underchassis headers vers fenderwell ones? Also, I'm putting an "X" style exhaust system from the headers back with low restriction mufflers. I'd like to go to the larger 3" pipes rather than the 2 1/2" original pipes. Is there any performance loss by going to the larger exhaust pipes?

Any info or opinions will be greately appreciated.

jim coster, pittsburgh, pa
aka "jimharvard" at FABO