You can use rivnuts, one of FastJeff's favorites, through the aluminum angle and then use flathead machine screws to hold the cockpit sole plywood down. This is better long-term than simply using a bigger screw. I install the rivnuts using a poprivet tool. Mcmaster.com has them. You might consider using neoprene tape over the aluminum to give the plywood a better surface to sit on.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida