I think looking at the options combination you could get a 44, with torqueflight non sure grip rear end, and that would get you the 22" radiator. Its not common, but they pop up every so often. Once you add a four speed to the 375 HP 440 or to the 426 Hemi, you get a performance package option (Trak Pak or Super Trak Pak) and that gets you a 26" radiator.

The opening in the floor pan above does not look stock to me. The four speed opening was stamped. The hump goes over the opening with about 1/2" to 1" of additional coverage over the opening. The hump is stich welded from the inside of the car in about 6 to eight places with about 2" long welds.

All of this is assuming you are talking about 68 and 69 B-bodies.