How are you getting around the smog regulations where you live? In Ca, I have to swap a newer motor than is already in my truck and have to preserve all of the emissions stuff from the newer motor, if I want to change it. I wonder if I could pull my engine after installing a motor, get the truck registered as an electric, and then go back and install a little diesel engine.
If you really leverage the batteries in your setup, and enable regen, then you might be able to only have a small engine, like one of the kubota tractor motors (or just use what you have). I'm thinking that you could use the motor with or without the engine to help with acceleration, and, any time you are just cruising, you could use the engine for forward driving power and crank up the regen on the motor for the next time you need to accelerate.
With this kind of setup, you'd set up your throttle so that it limits out with the engine 1/4 of the way through and then activates a potentiometer to start applying electrical power to the motor. any time you are at cruising speed, your throttle is backed off into the first 1/4 range and this activates regen on the motor until your batteries are charged. This might be a seamless way to keep your engine at peak efficiency/high load and automate integration/function of the motor with the engine.
Ohhhh... I don't think I like the shift knob rotator motor throttle idea, unless it has a cutoff switch. I can imaging running into a corner hot, grabbing the shift knob to drop down to 2nd gear, and accidentally applying power to the motor, breaking the rear end loose or something...
Might need to find a way to do it so that you don't apply pressure to the throttle when you shift. Also, I've heard about residual weight from resting a hand on the shift stick causing wear to trans parts... Not sure about this, but it's worth considering.