i think you should go for 2.5" currently i have 2.25" and have noticed no power loss or anything, so maybe a little bigger won't hurt. I think that you choose the muffler, and then go to the muffler shop and have them fabricate the rest. My '65 289 was a single exhaust...i purchased a set of glasspacks and they fabricated the rest for around $100
College Station, TX
College Station, TX