Wallace....there are other possibilities...so you could consider
1) support the lower hull in 3 or 4 wooden cradles, securely mount these to a substantial building board...you may need to drill for 3 or 4 screws through the hull & secure the hull to the building board or wooden cradles...don't worry ....the holes can be filled later
2) add & epoxy bond a wooden coaming [say 10 mm high x 6 mm wide] at just below deck level yes...around the entire internal port & stdb sides of the hull & from bow to stern
3) design & cut a
true profile deck plate 4) mechanically distort the hull until this
true profile deck plate fits into position
5) gauge the accuracy of the alignment fix...this can be as complicated or simple as you choose
6) assess what long term physical access you need into the hull from bow to stern & the cut these access provisions in the
true profile deck plate Remember that a twisted GRP hull when straightened will create redisual stresses ....so consider placing additional internal hull frames to suit the corrected profile & epoxy these in place
7) naturally one of the last acts is to epoxy the
true profile deck plate back to the wooden hull coaming.....
All very similar to the comments from tigertiger.......

Derek