well the Chysler straight 6 225's are almost bullet proof. i had one in a Plymouth Valiant, went 250,000 miles before having to go into it. i have no idea what the fuel consumption would be. you need to fill up, take a cruise, and then top off again. that is a big boat to guess consumption.
Sure those are "slant sixes"? I don't see one of those making 225HP... I think they made closer to 120hp.
Anyway, T'daddy is right you'll just have to test it and find out. That's a heavy boat and I'd imagine the fuel consumption is going to be pretty stiff. those old engines were not fuel efficient to begin with (compared to today's engines).
And for gas engines that rpm number sounds way off for that speed on a 9500lb boat... I'd think 8-9mph tops.
