mounting a t top on a 78 Mako

this is another project I'm on, So I'm gonna install a t-top on the mako, 20 feet, I plan on using backing plates, which means I have to drill access holes. I'm thinking 3" hole I can just screw a deck plate onto when I'm done. What I'm wondering is if I'm going to run into a wall of foam when I drill into the deck, I had an aqua sport before and it was hollow space under the deck, but thought I'd check here to see if anyone knew for sure.

fiberglassing in stringers might work.anybody else?

not sure what you mean,...is that attaching the t top to the stringers? If its foam filled it would be hard to see where just where the stringers are. I'll find out whats in there when i cut the hole but if its all foam filled, i have to dig out the foam to fit the backing plates under the base of the t top. If theres no foam, I can cut one hole and reach in both directions, but if Im going through foam, i think I'd have to cut access closer to the mounting, meaning a hole for each leg. I'd like to put as few holes in the deck as I can.

I meant installing extra stringers in flooring glassing them in.Then repainting floor but disregard because you dont want holes in floor.I would wait for more replys.

OK, ended up using stainless toggle bolts, manufacturer said screws would work, as its also attached to the console, but I felt better using the toggle bolts.