Proper Exhaust Diameter fo a 4bbl 225?

I agree with killer6, the problems your tune not the small exhaust. But I would go with a single 2.5" with a good muffler.
Put long secondary pipes (2" or so diameter) on those headers with a paint stripe down each one. Take it for a spirited drive - the paint will burn off part way down the secondary pipes. This is where you want your collector for the single outlet.