Re: 1979 Imperial
Man, that stinks... And that transom was so puuuuuurdy too --- looked like caramel cake! I'm with Coonie (that's jb)... all may not be lost.... Since you have got an outboard and the you are not needing super-precise specs (for a key-hole, etc...), all you are after is strength back there (for the weight and push of the o/b)... Thus, I would see about just just flattening it out with planer/sander, etc... and then if there is still a gap, fill it in with pb.... (of course, that sounds like a bit of work and it may be easier just to start over). I wouldn't clamp it - not only would it look bush league, but it would eventually want to delam on you as it fought to go back to its original (out of the mold) shape...that's why I had to cut mine out and start over...
Just to be sure.... you are sure the wood is warped and you are not dealing with a curved transom, right?
If its any consolation, I have been working on my transom since November... seriously, its been that long.... So, it sucks to be you right now, but probably sucks worse to be me.. :facepalm: