Bob,
congratulations with the BJ, it will beat the "jeep feeling" big time

I was kicking myself i ever bought a jeep will be the next thing you writhe. lol
The frame fj and bj 40/42 are the same.
The converter you can buy aftermarket but normally the guy before you has already installed one. (Radio's on 24V are very rare),just consider what kind of Amps you like converted (fridge,CB,Radio,.....)
Good luck with you're frame, and welcome
By the way we love pictures