wow ! you guys are still "yoots" !!! laugh2
over the [many] years my brother was growing up, and i was a junkyard owner, all the 318 cars he had retained the factory manifolds, but had muffler shop duals made with 2 1/2" pipe adapted to a short section of pipe off the manifolds after everything that was in the road cleared.
sounded great, using aftermarket short turbo style mufflers.
i tweeked the distributor just a bit on the weights and springs, and those 'ol 2bbl cars ran really well, plus got good gas mileage to boot !
beer