You could make the sliperential work with one of the custommade centerdiff mounts from Gumleguf. I am not sure if you will be able to find one, as it was only made in a very limited batch (about 50 I think). It is made to use with the RC-Monster motormount and a centerdiff.
Maybe Gumleguf can chime in on the possibility of making more?
To expand on what BlazerSS wrote, you can make a newer tranny fit an older flm chassis with an adapter plate flm use to make. Actually, the newer tranny would fit almost any older chassis with the adapter plate flm used to make.
It is not a direct bolt in fit.
FLM doesn't show the plate on their website, but you may still be able to acquire one if you contact them.
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It's "Dr. _paralyzed_" actually. Not like with a PhD, but Doctor like in Dr. Pepper.