If I recall correctly... the earlier 8274s did indeed ground thru the mounts. The motors were securely bolted to the housings, which in turn were bolted to the bumpers, which in turn were bolted to the frames... etc... anyhow, Warn discontinued this setup in favor of a ground strap utilizing the end bolt that secured the motor to the housings. The ground strap went from the end bolt to the battery. Both of these styles are quite ancient. All of the more modern 8274s have a dedicated ground off the motor. Each 8274 motor should have 4 studs. One is common, one is forward, one is reverse, one is ground. If this winch has only got the three studs, it's a very old winch, and you should take that into consideration.
All winch motors should have a ground strap from the motor directly to the ground post of the battery. Period.
Although you may be able to run a winch by grounding to the frame, and from the frame to the battery, that is NOT the proper way to run a winch motor.
Warn winches can also be judged age-wise by the prongs on the winch controller plugs. The earliest winches had a cable which operated the clutch on the side. They are normally refered to as an upright 8000, not the 8274. The early 8274 had three plugs on the connector, which were about an 1/8 th inch round. The mid 8274s had three plugs, but the plugs were only about a 1/16th of an inch thick. The latest 8274s use 5 plugs, also about 1/16th of an inch.
again, this is all typed from memory, and could be completely wrong.
T/ out.