I am building a B&B with a 460 BB and have been following the radiator posts. Here is my research to date:
The 67-69 Mustang Radiators will fit, with mods, but only cool ~250HP.
The 67 Mustang/Cougar 390 will fit, with mods, and cool about 350HP but nobody makes one to the original 23-1/4" core width.
The 68-70 Mustang 390/428, 24-1/4" core width are to wide and hit the body.
All of the above need bracket mods because the radiator must be supported by the tank on the bottom.
The 77-78 Mustang II radiators have the brackets and support structure to mount to the B&B front clip. They are also narrow enough to fit inside the body.
US Radiator will custom build a Mustang II radiator that will bolt right up to the B&B brackets and with
the bottom outlet pointed up! They also made the outlet the right size at 1-3/4". Only in copper/brass but can handle just about any HP. Part # 53160.
http://www.usradiator.com/catalog.htm
4 Row ~325HP $290
4 Row High Efficiency ~400HP $358
Optima (6 row equivalent) ~475HP $418
Tripple Flow (7 row equivalent) ~???HP $449
I have one on order, 2-3weeks, and will post how it fits.