Re: Removing back to back seats with pedastal seats
What type of seat base are you going to install, base that does not require the big center hole, or a base that needs the 3-4" hole? Also, how thick is your deck? You said 1/4" w/fiberglass, but that seems awful thin. Usually deck plywood is 1/2".
If using bases that require the big center hole I would add in a backing plate under the deck. If the base does not require the big center hole, togglers would be the best way to install the base.
Springfield has
under deck base rings that might work well
IF you have enough clearance below deck,,, and there are no obstructions.
If you hit a stringer, pray that you hit it in the center and not off to the side. (IE, screw won't grab and toggler won't fit) If screwing into a stringer, pre-drill the hole, jam some epoxy into the hole, apply a thin coat of oil or wax to screw and run it into the hole and let the epoxy setup. Then remove the screw and install the base. You could just epoxy the screw hole and run a bare screw into it while installing the base, but I like to have things removable. Removing an epoxied 1 1/2" screw might be difficult.
(FWIW, Crestliner installed the seat bases in the 1/2" plywood decks of my boat with 1/8" aluminum backing plates and #14 SS screws.)