Back to concerns about the strength/weakness of the custom coupler....
The thinnest diameter shaft in the chain isn't the tranny input shaft, it's the splined shaft welded on the end of the Lovejoy coupler that slots into the motor. Its neck is only about 2/3 the diameter of the input shaft. Compare (click = zoom):
Still, if the transmission shaft is 100% over-engineered, then we should be OK with 66% over-engineering
Actually I'm pretty sure shaft strength isn't linear with diameter, so the over-engineering probably doesn't scale like that.