My 30 quart aluminum pots works for my mash tun (I BIAB so it is also my boil kettle) so I don't see why you can't use your 50 quart. You will have to insulate it somehow so it keeps the mash the right temp.
I do BIAB also so my mash tun is my BK. I bought some Reflexit insulating blanket (sort of a foil-faced bubble wrap) at Home Depot. I wrap 3 layers around my pot and secure with velcro tape. It holds my mash temp within 2F for an hour. I remove it when I'm ready to boil.