single exhaust for 318...what size??

The single exhaust is a good thing when done right.

If you want to do a little custom work, try this:

Run your header collectors down to 3" pipes and into a 3" merge collector.
Run the 3" pipe to a 3" Flowmaster and then out to the left. It will sound good and perform well. Very simple system. The bigger the muffler the quieter it will most likely be.