BJ40man,
in your signature line, it says your 74 BJ40 has a 3B.
Did you bolt the 3B engine to the tranny that you B engine originally came with, or did the 3B come with a post 08/80 transmission (and split case)? Do you know what transmission you have in the HJ45?
As you stated, the H and B transmissions won't bolt up due to the B having a 1.5" longer input shaft and 21 vs. 10 splines.
It's tough to say; popular opinion regarding split cases seems to be dominated by our American friends (no harm meant) who didn't get these (at least in their pre-10/82 H42 4-speed's). I would suspect if you have a split case on both your B and H drivetrains and they are both from 1980, they should bolt up. You can in fact mate a B engine to an H or F transmission (vice versa) by rebuilding the trasmission with the proper input shaft)
However, I know that the split transfer case from a (3B) H55-F (10/82 - 10/85) will not bolt to a split case'd (3B) H42 (8/80 - 10/82). (I know you're talking about an H to B swap, but it goes to show that not all Cruisers were created equal!!!)
You might try 'Crushers' (on this site), as he's swapped 2.276:1 split case gears from an H42 into an H55-F split case that normally came with 1.959:1 gears (as well as many nice restorations). He may know of actual transfercase compatability.
You may also try the TLC FAQ (my cousin's site):
http://www.birfield.com/tlcfaq/
I couldn't find anything concrete in regards to H & B series trannsfercase swapping, but it has LOTS of info.
Good luck (and post it up when you find out)!