IMHO, early flatbacks, AS or PL, as well as the 12 degree deadrise hulls, were pretty much designed with the max horse available at the time to probably around 150, more likely 75s through 90s. The weight of the 3 cylinders was so much less then even the lightest I/4 (Yam and Suz 115/130/135/140/150s) and DEFINITELY much less than either the Opti or Etec V-6 150.So, once again, IMHO, we are looking at slapping a motor on a boat that, 1) was never designed for it, and 2) those motors weren't even a twinkle in their Daddy's eye when those boats were built...So, all that being said, and getting rid of as much accumulated water weight (read that saturated foam and transoms), and shifting some of the above decks glass weight forward, as well as any tanks...150 Opti is my choice...JMHO...
what year is your boat steve? it seems they changed from total flatback to that slight v with those mini "strakes" around 74 or 75. my friends got a 73 and its still total flat. and its got the aquasport tumblehome to it, this ones almost straight up and down.