Just wondering if I would loose a lot of power and torque going from cheap Hedman Headers to Magnum Manifolds? The application is a early LA 318 with the small Summit Cam, Performer Intake, and 600cfm carburetor with electronic ignition. Pistons and heads are untouched. Car already has dual exhaust 2.5" with Dynomax Ultra Flo mufflers. The car has a 2.76 axle ratio. The main use for partial daily driver duties and road trips. Also, which Magnum Manifolds will work for a 68 B Body application? I have access to a Ram truck, a Ram Van, and several Dakotas mainly 94-98 variety.
Not a fan of headers so, I'm throwing it on the table since to me they look out of a place on a small block B-Body plus these are starting to leak.

You'll lose some but not sure how much. The early mag ex manifolds (92-early 93?) have ~2&1/8 (or maybe even 2&1/4" I'd have to measure mine), outlet & all the later ones have 1&7/8" outlet, quite a difference. they have the same outside dimentions & I believe a different casting #.

going backwards.

if you use them,search out the early 92/93 manifolds for best size and flow.

I would replace the used set of headers with a new set of hedmans,newer ones have the ball cone collector hook up which has no gasket to blow.

http://www.moparmusclemagazine.com/techa...t/photo_17.html

on the 300hp magnum crate motor, they found about 14-16lb-ft difference in midrange torque between 360 or 340 manis and hooker comp headers (essentially the same primary tube design as the hedmans) and less (4-5hp) peak HP difference