Heavy lay up thats a good thing right?
Depends on you point of view. Heavy = thick, strong, but heavy. Good cruiser / lake boat. They made 4 versions as far as lay up, H = heavy, M = medium, L = light, and SL = super light. SL was only for the race boats.
There was also many different lengths of Sangers, 17' 10", 18' 3", 18' 6", 18' 10", etc, etc. As well as different bottom designs among the "flats". There where true flats, runner bottoms, and the gullwing. That one looks to be a true flat.
Hers an idea for you. Sense you going to chop the bottom off and build a jet friendly bottom, why not get creative and build it into a tunnel hull ?
And as far as that JE pump you have,

You would be way better of finding a "normal" pump. That insert pump is going to turn into a real pain in the azz to get right. You might get ahold of Tom over at Jet boat Performance though. Maybe the next time he chops up a boat to replace one of those things with a standard pump set up, he can save you the insert section. He might even a section sitting in the trash pile.