:WELCOME:
My very first boat mod was a 12' Fiberglass boat too. So here's my 2 cents worth...
1) Your boat is already heavier than hell so the goal is to make it more functional for your needs without making it too much heavier than it already is (draft is key).
2) Imo, you don't need a floor in that boat because the way it is now it's easier to clean (maybe a good coat of quality paint if you want).
Drainage holes between the seats (using pvc pipe) & a drainage hole (or bilge pump) out the back is a MUST (unless you're a fan of bailing).
3) MOST IMPORTANT: BEFORE you start ANY mods, check out the strength & condition of the transom. make sure it's solid with little to no rot. IF it's badly rotted, my advice is to dump the boat asap & get an aluminum boat. I know that sounds harsh BUT, putting a new transom in a fiberglass boat (even 1 that small) is an undertaking that's in short, a MAJOR pain in the arse. And just for the hell of it, I'd put some sort of bracing on that left & right side of that transom (inside-natch!)
especially if you plan on using an outboard gas motor on the back (suggested 5-10hp max if there is no Coast Guard sticker stating otherwise).
4) If #3 doesn't apply (I.e. no transom rot), the best idea for designing a front deck is to make/cut out a full size cardboard template first.
Figure out where you want things (livewell, storage, seat base etc..) to go where.
5) Once you have established all of that, I'd use 1/2 treated plywood for your front deck with some bracing underneath (the bench seats look solid-use them to support most of the decking). Important note: Before you make ANY decking permanent, check out the stability of the boat (I.e. set the decking in the boat, float it & see how stable it sits in the water with you sitting/standing in the front). How wobbly or secure does it feel?
6) How many people are going to fish out of the boat on a frequent basis? This will determine if you feel the need for a rear deck on that boat or just a rear pedestal seat. Well it's a start & I'm sure others here with more experience than I will chime in with their various suggestions (especially about adding extra foam for flotation should God forbid the boat take on water).
By & large were a helpful group of chaps. :lol:
Good Luck!