7 1/4 - 5 on 4.5 BC A body

I went around on this with both cars. IMHO, it is easier and more cost effective to get a 7¼ BBP assembly than to try to convert an existing SBP to BBP. Avoid the 2.21 and 2.45 gear sets. They require a different housing. While Mopar made a number of different gear ratios for this axle, I haven't seen any aftermarket support for the 7¼ these days.

The 8¼ and 8.75 are good, too, just remember to get the spring plates and clips (u-bolts) with them. The tube diameter on the 7¼ is 2.75 inches, the others are 3 inches in diameter.