Depends on condition of the interior, in particular any water saturated foam under the deck....may make it much heavier than you had planned resulting in so so performance with the boat full of folks and gear. Slant Six???? Inboard? OMC used their own 2 stroke or the little Buick Hammer Head V8; Mercruiser used Ford or Chevy (or their attempt at an I4), I don't remember the engine in a Glastron with a Volvo outdrive, but it wasn't a slant six....all I/O propulsion and had a hole in the transom that you would have to close up. Since you would have to redo the transom I think I would go that route....replace the transom, cut down for an outboard, preferably 20" or so....20" would be the most available.
If you are thinking about a jack plate on the transom, think about tilting up problems....where is the power head going to go when raised...gotta have room for the lower unit to get out of the water and keep the skeg off the highway when towing.
