Andy, I'm not sure what design you are planning for your winch, but I think you will need a much lower gearing than those motors you have.
My winch (admittedly untested on a new boat) has a gearing of 3000/1 which will give a turn speed of about 5 turns per minute. Lower gearing will tend to spool out when under tension.
Also remember if you use it for competition tug towing you are only allowed to spool in or out to adjust your line length, not to actually winch the vessel.