If it were me, I would have it welded completely on both sides. My buddy's Crestliner and my Seaark are done that way, but they use an extruded aluminum keel, the two sides of the hull slip into a groove and the entire thing is welded up tight. There are probably many other builders that do the same thing.
If there is a way to lift the floor, I would also check the area of the leak for cracked welds in the rib supports.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner