Ramcharger exhaust 2-1/4 duals or single 3"

My stock 87 Ramcharger 318/4bbl with manifolds. Currently has 2-1/4 duals from the manifolds, 2 cats and turbo muffs that exit behind rear wheels. I am in a quest for the best fuel mileage out of a non OD and carbureted beast. I play around and have done great with it so far. 13.5ish mpg. If I recall when I put the duals on it I actually lost some mpg in doing so.
My thoughts today are if I put a y pipe in behind the cats and run a single 3" back. I'm thinking that if I do so , I could possibly increase some backpressure and maybe some torque and gain some mileage. A single 3" should still flow well enough for a stock 318 and not be too restrictive.
Thoughts?