Double-ended motors are not overly common but they can be had.
I'd put the electric behind the tailshaft of the tranny. When I want to run straight-electric, I kick her into neutral and shut down the engine.
If you want to use the Benz engine, get the Benz tranny and modify the frame to support. Match a DIN block to a SAE bellhousing makes tax law look simple.
The 616 engine should bolt right up to the four speed out of a 240D. Nice thing about this rig is that it will be running just fine when the rest of the truck rusts out around it and you could put the engine/tranny in another truck and probably outlast it. One caveat: Benz parts are uber-expensive. Pay more and get a good engine. Hit
MercedesShop - Mercedes Benz Parts and Technical Information for advice. They are seriously cheap-Charley and know all the good parts sources.
Ever thought about a late-model (1986) 5.7 GM diesel? By then GM had cured the head gasket leakage problem. Its a bolt-up on your T-5.