Re: 1975 Crestliner Muskie Restoration
Take them out 100%, grind the area smooth to good glass and rebuild from scratch. You want ALL your new work to become a part of the boat. The only way to ensure that is to remove everything & redo it.
I'd recommend you look for MDO, Aurauco, or ACX plywood for your stringers, transom & deck. ACX is the lowest cost of the 3, it's exterior exposure rated plywood, and made w/ waterproof glue. The A face has few surface defects (if any), the C face isn't but could be used for the down, back or non-visible side, and the X stands for exterior grade. BCX is much more common, and may be easier to find.
Here's a helpful graphic for how to put them back:
"Fabricating Decks, Stringers, and Transoms"
Compliments of WoodOnGlass ^^^
I'd use plywood before I used dimensioned lumber. It's stronger at a given thickness & length, it's more stable (tends not to be warped or twisted once you get it home, and a 12ft 1X6 is likely to be cupped, warped & twisted) and should be cheaper to buy in the necessary quantities. If you need stringers longer then 8ft, you can join 2 lengths together to get the necessary length. We'll get to that, when you're closer to doing the put back.
For now, spend some time reading thru some threads, lots of good info in most of them. It doesn't have to be a Crestliner, generally fiberglass boats are built similarly....
If you get to cutting & grinding, be sure to suit up, and wear all the correct PPE..