A Lovejoy coupler should work fine in this situation. It will withstand slight misalignment also.
The Rittenhouse catalog has a Honda with built-in 6 to 1 gear reduction.
Or...with pump preinstalled for a few dollars more.
And yes, it may be necessary to mount it on a pallet--or pallet that you cut in half so that it fits on your tractor, and yet will fit on your truck or trailer.
Do you have forks for the front-end loader so that it can be moved easily? If not, you may need the tank and motor/pump separate so it can be moved in two pieces by hand.