Thanks GlennD, that helps a lot.
Sounds like my best bet is to sell the original OEM EVSE and look for a used 40A cable. The only thing left then is to decide whether to run a 50A or 40A circuit. I think I have a spare 50A breaker already so I guess that's an easy choice.
Another question, if you don't mind.....
I read somewhere that the NEC limits the length of the cable from the receptacle to the EVSE box to 18". Is that right? I have a used 14-50 plug/cable that is about 3' long that I'd like to use. Worst case I can cut it down and re-crimp the terminals but I'd rather not unless that is required.