Proper Exhaust Diameter fo a 4bbl 225?

Hi all,
I recently had the 225 in my 65 Valiant 100 (engine is a mid 70's) rebuilt with a 4 barrel carb, performance cam(not sure of the specifics) Clifford intake and dual exhaust headers, all of which feed into the original exhaust pipe. I'm noticing a dip in power when hard accelerating especially from a stop, occasional backfiring and a loud 'blatting' from the exhaust when accelerating at speed. I'm thinking that I likely need a more substantial exhaust pipe than the stock 1.75 inch single pipe, but could this just be happening due to early days after the rebuild (engine hasn't yet been readjusted after the 500 mile break-in)?