early A body exhaust EXPERTS!

IMHO - single has a ton of advantages. Less weight, easier to route, better scavenging, less money and on an early A looks stock(ish).

This is what I did on mine, Doug's headers into two 2 1/2 inch pipes, Flowmaster Y-pipe into a single 3-inch muffler and tail pipe. I even bought the factory repro tip from Accurate Exhaust.

The crossover happens before the transmission crossmember so I added a flange after the muffler so I could easily drop the system to pull the transmission out.

You would have to have a very high strung motor to see any difference in your timeslip over dual 2 1/2-inch pipes.