The RAD 4-speed in the 302 powered MII has a smaller bellhousing than a "standard" Ford 4-speed. It also uses a smaller flywheel, clutch, etc., and it is balanced at 28 oz. vs. your latemodel 5.0L balance of 50 oz. You really need to skip the RAD.
If you are hurting for funds, and your 5.0L isn't too hot, and you don't do any drag racing starts, then the T-5 behind a 3.8L V-6 latemodel Mustang would hold you
For A While. Probably a lot longer than a RAD would as the 3.8L RAD has a higher torque rating.
You may have a shifter location problem with the T-5 anyway. These guys have a shifter relocation kit ($$$) under their Products section:
http://www.mn12performance.com/
Don't feel bad though...the Mustang II has the same problem when swapping in a T-5.
If you are going to sell or part the CBII, please let us know here. There are so few left that it would be a shame to lose another.
78CobraII