[ShadeTreeMech]

I wouldn't rule out making it front wheel drive because of it being too hard.

Simply use the original 5 speed tranny with the motorcyle engine, and then you will be using the existing design when it comes to driving the front wheels. And with the lesser HP torque steer is irrelevant.

You could attatch the drivechains of both engines to a common drive shaft that would then go to the transaxle. When shifting, you could use the car gears, or make it where you can use the motorcyle transmission in the cab and essentially you could have a multitude of gearing ratios. If you want to disengage one engine, use the clutch cable to dissengage one engine from the system and have a way to lock the clutch into the dissengaged position. If I'm not mistaken, the thrust bearing on a motorcyle is immersed in oil so there shouldn't be any extra wear due to the clutch being disengaged for extended periods of time.

It also takes some of the guesswork out, since you are reengineering less of the car.