I bought a 2-speed electric motor and switch for my 57 GMC. Got it from truckandcarshop.com $180. It fit perfect and is very strong. You cannot stop it with your hand. Not sure what brand they sell, it may be a Counterpart.
If I had a complaint, it would be that the switch is a little hard turn between positions. But I expect that to get better with use. At least it is chrome and looks like the original switch.
Have you tried un-hooking all the linkage from the motor, to see how strong it is then? Might be some binding somewhere?
