Good advice from 46gmcpu.
Do you have an SM420 4-speed transmission or the standard 3-speed transmission? I would not try to put a horse&trailer with the 3-speed (1st gear would be too "high").
I would not use a bumper hitch.
There have been a few recent posts regarding
frame-mounted bumper hitches (and, I think
Deve has a home-made hitch and
plans for that hitch).