Okay, as promised, as soon as I was able to get an RC Humvee I said I'd run some tests.
I took one that had been beat to death as a demo model (when it ain't your toy, apparently it's "cool" to trash it). It was a dead fish. Turns out the front motor (it's 4 wheel drive) broke (not fried) after repeated full-throttle runs straight into immovable objects. I was actually surprised it lasted as long as it did. Thanks to some ham-fisted John Q Publics, I had a test piece.
Complete removal of original 21st chassis. Trimming down of the battery tray and cutting out some of the Hasbro center resulted in a good fit. Both toys are VERY close in dimension. You can see that by the 21st slantback that screws perfectly onto the Hasbro body. No, nobody copied anyone, these are about the only two holes that match anywhere on either. Very lucky for us.
Still has that Monster Humvee lift-kit look, but I'll try and lower it more later. Right now, this thing zips around my workyard like the dickens. I'm giggling my asss off. With both motors, I always had trouble keeping control. Now, it has plenty of guts with just the one motor and is easily handled by even an incompetent R/Cer like myself.
In a couple weeks, I'll start customizing the 21st chassis itself. With a bit of cutting and fitting, the stock 21st toy will accomodate figs easily. Stay tuned.