64 coupe resto with 396ci sb blueprint motor and 700r4
best performance exhaust options.
Megaflow mufflers currently out of stock and looking for other options along with possibly changing to long tube headers although mixed info if headers actually have much advantage over my 2.5 manifolds

 

I assume you're talking about MagnaFlow mufflers in the stock location. Have you considered nest of snakes side exhaust headers? I have no data but I always considered that style to be more efficient than under the car exhaust systems. I have run both nest of snakes side exhaust and factory side exhaust and the car seemed more responsive with both as compared to how the stock under the car exhaust was.