checkout www.fordpowertrain.com, sounds like you need at least 1 3/4". They had the a great longtube solution for me on my 67 with 302 and AOD combination. Ground clearance will be excellant, they cost a little more but with your combination it probably makes sense.
Well, actually it will be cooled with a methanel injection and 20psi. Thats for the future, I have to get the bugs work out first. I will look into the headers, thanks...