Before you "can realistically expect to measure gas mileage"????? I'd be more worried about fire than I would be gas mileage. Gas leaks are not something to ignore. A gas leak and a backfiring engine are a recipe for major disaster. Take it from someone who's been there, watching your hotrod go up in smoke is not fun.
About the idea of side pipes. I put them on my AMX way back in 1970. They look cool, and sound awesome, but if you're going to use the car for city driving be prepared in inhale a ton of exhaust at every stop light. And be sure to tell your passengers to watch their step when they get out, your friends will not be happy with the burned calves that are inevitable.
BTW your car looks very cool.