What I am finding out is that these transmissions use a SAE#2 Clutch Housing. I found a chart that gave all the dimensions, and when I measured my 454 it isn't even close.
See:
http://http://www.sbmar.com/Maintena...ings_Jan07.pdf
Looks as though the SAE housings are rounded, where the GM is more irregular in shape. I am guessing that when Chevy mounted these on their medium duty trucks, they used the standard bell, and then a plate type adapter.
Eaton Fuller finally emailed me back, and can't really give me much info, except that they thought this was a very interesting project. LOL They did say they were going to refer my info to the Chevy Design department in hopes they can come up with how this was done. My research has now led me in another direction that possibly has ready parts available. Seems the ZF-S6-650 can also handle a lot more input torque than the NV4500 Sure is getting old trying to find a combination that is super strong with readily available parts.