welcome to owning a tahiti. i just got done with mine and handed it off to its new owner.
tahitis arent known to be the BEST made hull in the world... and def not the strongest. my deck was very similar. i could literally see it moving, and dont even think about walking on it... but after doing full length stringers and adding a bulkhead mine felt WAAAAAAAAY more solid and it rode 10x better and you could walk on it without any creaks or cracks.
everyones probably sick and tired of my pix... but here goes...
foam is cheap, its easy to cut and makes great templates!!!

once u have your template, it gets cut into 3/4 ply. see how easy all those random angles were! make sure you have like a 1/8" gap on the sides. you dont want to wedge it in there as boats flex. if u have a rigid bulkhead in there and the boat flexes it will crack the gel, etc.

use good stuff to put it in. west systems is about as easy as it gets. if u can paint, u can glass.

it gets "tabbed in" where u take like a 6" wide piece of fiberglass and fold it in half and it gets stuck to the piece and the hull all the way around. inside and out!

carpet goes in to make it pretty.

go boating! enjoy a nice strong deck with no flex.
