Interesting thread, and great work.
I bought a 289-style hardtop a few years ago. To fit the 427-style body, the roll bar and its rear leg, had to be cut into 4 pieces, new rear skirt to fit the 427 bodyshape and glassed back together. Prepped and painted it with 2k paint. Just finishing off the fixings, and will need to fix a few paint issues you can see here:
https://www.youtube.com/shorts/0B_9zDOgmKk
Heard these hardtop are notorious for leaking at the middle of the windscreen. To address this I bought a pair of standard soft top hood bows, pinged off the locator pins, and then cut a piece of curved channel to fill the gap between the 2 bows. I used polyurethane sealant/adhesive to fix these pieces together and to bond the hardtop to the bows. Some grp ribbon still to be added along the top to make sure it doesn't blow off t high speed.
If anyone's interested I can post some more, or start a new thread.