B4 you that 4 blade a prop (I wouldn't for your application) test more with the ones you have. Since they are a far amount less than 7" diameter and your revs are way up there (with excessive slip from your descriptions) I would try deeper (drop 1/2" same tuck at first) and see if that holds better and the revs tame down. You should look for about a steady 7100 rpm and with a 12" pitch that should get you near mid 60's for the 302. A 7 x 12-4 blade might work but probably will be slower than a 3 or 2 blade but it will definitely drop the revs.