Brain training

I think I've found a possible approach to solve the issue with the coupler / adapter for the motor and transmission:

1. I require a shaft for the motor. On one side it has to be splined (the half that goes inside the motor) and smooth on the other side with a key shaft (the part that sticks out).
2. On the part of the shaft that is outside the motor, I'll attach a taper lock (aka quick detach bushing).
3. A washer between the motor and the taper lock will keep the turning parts away from the Motor front.
4. On the transmission side, I'll use the inner part of the clutch disk to attach a machined ring to it.
5. The machined ring and the taper lock will be connected with bolts.
6. A 20mm thick adapter plate will be machined for the motor and one 20mm thick plate will be machined to fit the bell housing of the transmission - both aluminum.
7. The motor will be mounted to the transmission, both placed upright and then I'll spin the motor at different speeds and with minor adjustments, I'll try to get rid of misalignments / vibrations.
8. Once the vibrations are (hopefully) gone, the two plates are clamped together, some holes will be drilled and the plates bolted together to lock them in place.

The taper lock on the motor shaft and the washer will make sure, the shaft doesn't move inside the motor. The motor shaft touching the transmission shaft ensures that it doesn't move out of the motor. So the coupler is fixed laterally although it's not tightly attached to the transmission shaft. There's some room for imperfection because the nut in the clutch disk isn't a perfect fit on the transmission shaft. So the whole connection is not a tight fit between the motor and the transmission. Hopefully this prevents damage to the motor's bearings and splined shaft.

So the only custom built parts should be the motor shaft and the adaptor ring. But as they're pretty simple, I think it won't be too expensive. Hopefully I can get the measurements of the bell house from Ford. This would help a lot to align the two parts precisely.