Single would male more sense on an early A, but it's certainly doable. The cross sectional area of a 3" pipe is about the same as a pair of 2 1/4" pipes, but it will flow slightly better due to less surface area.
Thats what I was thinking, but not the surface area issue.
Flowmaster makes some good 2 into 1 transitions, but they are a tad spendy. Then again, you'll only be buying one muffler. I'd talk to a good muffler shop and compare prices with one of the pre-bent dual systems..
I run a 3" single on my truck with a mild 350 (Buick 350 GS Stage 1), and it is perfectly adequate and reasonably quiet. Should be fine for your mild 318.