7.5 mercury thunder bolt

i just got this motor and don't know anything about it other than it runs good. i guess after a few years on the shallow rivers around here it needs a new prop because this one is very worn. it has a 2 blade but i'd like to have a 3 blade but i don't know where to start. my first question is... what pitch? i called a few local marinas and was told a 9x8 but honestly i don't know what that means. i haven't had it on the 1664 flat bottom yet (going saturday) but it done good on the 1432 river jon last weekend. it wouldn't back up very fast at all but i figured the worn prop was to blame for that. how do you tell what year it is? only info i can find on the net says 7.5 was in 1976.

and what other horse power mercury motors is this compatable with?

I had a borrowed 7.5 Johnson 2-stroke on my 1542 last weekend. Pretty slow but better than paddling. Wouldn't get the boat up on plane. If you've got some weight in the bow and aren't in any hurry it should be alright!