The Mopar B and RB series have been used in the M715...specifically the 383, 400 and 440 that I know of...
NONE will mate to the stock trans unless you make your own adapter...there isnt one on the market anywhere that I have ever heard of or found...
Using the trans of your choice behind the 400 and then mating to either the stock NP200 tcase or a more modern NP205 makes perfect sense and is do-able...
I love the B series with its short stroke...3.38 inch and huge bore...the 400 has a bigger bore than a 440 by a little bit...with the right gearing, I think it would make an M715 fantastic!! Always wanted to do one but never seem to have the means...
Good luck!!
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!