What appliances and tools take that plug? 150amp welder or a full sized kitchen range/oven are the only things I can think of. Without a neutral you can have either 220v or 120v but not both.

The 50amp breaker appears to be oversized for the conductors. I assume you had a pump or air conditioner here before? That can be code compliant when the load has its own internal circuit protection. But not with the type of plug you are suggesting. Depending on what the manufacturer rated their wire for temperature wise. Makes me uncomfortable regardless.

I would think to toss it all out, 20 amp double breaker with a new run of 10/3, and use it for a pair of 120v circuits. Should be all the power you'd ever need for residential construction work. And if you ever need 220v you could easily rewire the plug to a 14-30r.

Anyway, with electrical it's usually safe to use larger wire and plugs but past 20amps you need to match the breaker to the load. So it's not valid to leave it as is and slap an adapter onto a 6-50p to hook up something smaller.