[Coley]

He is correct regarding the 'whip' associated with driveshaft length.
In 'engineering circles', this is regarded as 'critical speeds' relative to rotation and is dealt with by changing the diameter or stiffness of the shaft.
Noting that the smaller the diameter the more likely the unit is to 'whip' at high speed....so as you increase the diameter you lessen or remove the 'whip' in the driveshaft.
Your shop should be able to use past examples of trucks or vehicles with that span of mounting and use a similar sized driveshaft to overcome this....however you probably will be getting/making a new driveshaft to accomplish this.

All good
Coley