Update
Just about finished.
I got a welder to come out to the marina to fix the tabs on my samson post, and was told while helping him roll his gear down the dock that there's no welding allowed in our marina under any circumstances.
I quickly learned how to remove the samson post (which I should have just done to start with), and luckily he was able to bring it back to his shop and get it fixed up in a couple of hours. It was just bolted through the deck.
I re-installed the post, bedded with butyl tape, and then got to work fitting the piece of Starboard I'd templated off the original rotten platform. As you can see my skills are not great - could have used a good table saw and a drill press. It's in keeping with the general level of fit and finish of my boat though!
I convinced myself that the geometry would still work with the windlass 2" down, and that the 1.5" starboard shouldn't have any trouble resisting the relatively small loads involved - particularly after removing the samson post and re-affirming that it's really not designed to withstand anchoring loads - it's mostly support for the windlass platform and a hawse pipe.
After collecting the necessary hardware I got it all buttoned up today, except for one bolt which I'll replace with an extra long version to use as a grounding post to the bronze chassis. No ground terminal on this old Ideal windlass. I gave her fresh gear oil and a new set of friction pads; she seems to run just fine.
A couple of loose ends... my wildcat had been jumping links every now and then; turns out it's sized for G4, not the proof coil that came with the boat. Rather than dropping $400 on a new wildcat I'm going to go ahead and upgrade the chain, which needs re-galvanizing anyway. In the process of talking with CMP/Titan on getting a 1/2 drum delivered.
Finally, the windlass had struggled to turn the anchor up over the roller. I've attempted a number of simultaneous remedies (one of which being the aforementioned friction pads). I pulled the roller, cleaned and greased it. Thought about moving to a delrin unit but it's a really nice piece of brass, matched to a machined 3/4" bolt. The stuff I looked at wasn't as nice, so I'm going to try and make it work. I also replaced my anchor shackles with a beautiful stainless swivel that the guy at Downwind Marine swore by; it shouldn't hang up on the roller the way my shackles did. We'll see - it seems a bit small for my anchor shank. Have to think on that one a bit. Finally the retaining pin for my anchor used to fit through one of the shackles, but doesn't fit through the chain. I'll need to rig up something better - I didn't like the pin much anyway.
Thanks for all the help on this all. Feel good about the result. Please let me know if you have any comments or suggestions!