I've built 2 terns, and just finished 2 hybrid S&G's. In all four projects, I stitched the deck to the hull before gluing, just never trusted the tape approach. 3/8" on each side is really not that much, so as long as the overhang is the same on each side, I'd just carefully stitch the hull to the deck, flaring the hull slightly to accommodate the overhang on the deck. Start stitching at the butt joints by the cockpit for a frame of reference. You're better off with slightly more volume in the bow and stern rather than pinching them in, anyway.

I'd also cut out the hatches before attaching the deck, so you have access to the bow and stern areas.

Sounds like you're doing just fine. 3/8" is not atypical at the bow and stern. At least it's not an inch per side at the cockpit!

Jim