I would look for a rust free cab. Remember that if there is that much rust then think how thin the metal must be that hasn't rusted through. I you can find a big window then that's great but any rust free cab would be much better than a rusted one. It's not worth fixing.

1961 Long Stepside Apache 10, original 235, 4 speed and 3:90 posi totally stock (owned 12 years)
1966 Chevelle (owned 53 years)