I installed a leaning post on my 1984 Outrage 25. Odds are good yours is the same construction on the tank cover.
Look under the deck at a fuel access port. In mine, that area has about 1.25" of plywood built up beneath the fiberglass. If you use a screw, you don't want to penetrate the bottom of the wood, as your fuel line runs in that general area. You likely have twin 70 gallon tanks like in my boat - the single 140 gallon tank was used in later models.
I installed a Birdsall Marine leaning post, like
https://marineproducts.net/rocket-launchers-leaning-posts/?sort=bestselling&page=2I used #14 screws to hold the leaning post to the deck. I think I used 1.25" screws though they may have been 1". There are 4 screws per leg, or 16 total. I considered through bolting but decided against it and it would require pulling the tank cover to install as well as if I wanted to remove. I also used 3m5200 between the leaning post base and the deck. It is very solidly held down. I just did not see a need to through-bolt for this installation.