Forget 5200 ... you're NOT ready for that! You can't just screw it in, it will come out ... ask me how I know! My Capt's seat, screwed in from the factory (1992) sheared off at the base (screws pulled out!) and threw me across the helm cabin one day toona fishing in heavy seas! I woke up in the V-berth cabin floor and somehow had the presence of mind to drop the throttle before I was literally dropped to the floor ...
Anyway, here's what I did:
1 - Position base on floor, mask out with tape
2 - Check under deck for obstructions, lines, wires, etc.
3 - Drilled 3" or similar hole with hole saw (retract pilot bit!!!!!, or remove once started)
4 - One I had a hole to get my hand in with enough "room" to work, I could verify if there was anything a cutting bit would hit
5 - If no obstructions, use a jig saw with shortened blade if needed. Else use the hole saw, retracting the pilot bit, and cut a series around the circle you will need to do the next few steps, then even hend finish if worried about power tools ...
6 - My base was 10" across, so I used a 5" circle, as I wanted a good 2+" 'lip' of solid wood (each side) to hold the base, more is better, but you still need a good-sized hole to work through. Remember to dry test-fit all pieces, so if you need the access/work hole bigger, make it bigger.
7 - I cut hardwood 2" wide, 3/4-1" thick, and 6 to 8" long, whatever length I could get into the holes.
8 - Pieces were wetout with thinned epoxy and dried before install, then wetout and glued in place, plus two 2" long SS screws to hold in place whilst curing. Seal the sawn hole edges too with epoxy.
9 - Then the base was positioned over the hole to your initial marking
10 - Drill pilot hole, using base as a guide, into the stiffeners set under the floor. Remove base and drill through for bigger than bolt-size you will use. This is where the T-nuts will go.
11 - Install stainless steel 'Tee-nuts' for 1/4" or 5/16" hardware (I went with 5/16" for strength!) into the wood blocks from the backside. I used a series of large C-clamps to pre-load the and force their cutting bits up into the wood stiffeners
12 - Then seal (I would use BoatLife Life Caulk) and install seat and tighten tight (grease the bolts, using Triple guard blue-colored waterproof grease on threads). I used allen-head SS bolts and keep a wrench for them in my tool box.
13 - Periodicially check the bolts to ensure tight after some use and after a heavy seas trip ... FWIW I only had to tighten them once and it was just a bolt or two a tad loose or where the SS T-nut cut deeper into the wood stiffener.
See sketch ... modify pieces if your base uses an 8-hole bolt-pattern. But you want eh reinforcement to primarily go fore to aft, as that is the way the base will take the primary loading with someone seating, not that there won't be lateral stresses.
This is admittedly more work, but well worth it for both strength AND safety IMHO ...
