Yer more then likely gonna find a 292 2.5 inch exhaust manifold in a 63-70's C-40, 50 , or 60. The headers will make the biggest difference either way. And converting to dual shouldn't be to expensive...To convert mine to dual was like $240 and the mufflers were $75 a piece(Flowmasters are spendy anywhere). I think yer runnin glass packs...they are like $40 a piece and even as low as $20. lol If you do a dual exhaust dual 1 7/8 will be the largest you want to go. That will keep your torque, and leave it in the low range.
