Want the best dual exhaust known for my ''65 GT Commando

Can I make a suggestion? why not just get a pair of the 68-71 340 manifolds? They're available new and FIT and will outflow the stock ones by a mile and still look original cause they are....just not for your year. I mean if you're going with duals.....that the car did NOT have, why not? I can completely concur with your decision about headers.....I HATE um. They SUCK for longevity and ground clearance the really good ones SUCK for cost. the 340 manifolds are tight, but they FIT. there's a thread around here somewhere with somebody that has the later 340 manifolds on an early A, but I've seen the 68-71 ones on a car in person......my 1965 valiant I used to have before I put headers on it. a 2.5" pipe will come right off the manifold, no step down needed. That's what I'd do if it was mine.