There is a discussion of sorts on the Glasspar G-3 site about possible advertising pictures of boats with more hp motors than they were rated for.
The one a buddy has has a tag from the mfg. that says 270 hp. I know some of them had twin 135 Johnsons/mercs on them. I have seen twin 150 V6 Mercs, single 300 V8 Evinrude and everything in between. They work really well with a single small block V6 on them.
I remember the 18 with the V-8 on it. Great WOW factor. That says a lot for the design. That motor weighs about 545 which is still lighter than two inlines. jim
My '71 inboard with 318 Chrysler Marine engine is rated at 225hp. The Volvo-Penta 270 outdrive has the correct prop so when you are tacking 3 grand, you are going exactly 30mph. My nephew has it now in Murietta if you are interested in it. email me at facklerama@centurylink.net. David