I used to have a 28' Marinette made in the mid-seventies. The hull was very beautiful and flat-bottomed, would have been perfect for a river or lake. I considered putting in a jet drive, but ended up using the prop/rudder that came with it.
My '75 32' has a different hull. The forefoot is deep, designed to give better performance in a chop. This change was made to give better rough water performance, I would guess.
You should probably look for one of the shallow hulls instead of trying to modify a newer hull, which would be pretty expensive. The jet drive is a must, check out the Aussies who use jet boats on rivers a lot.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida