Pylon Attachment to the Hull Base Socket???

It does have the square flange and it is solidly welded to the pylon. I doubled the floor there so it is 1 1/2" of MDO - Medium Density Overlay. Sounds like the pin in the bottom would be overkill. Here are few pictures. of the pylon in my boat for test fit.

I've removed 2 different pylons like the one in the photo recently, but they have had the floor cover plate attached to them which through bolts to another plate below the floor and that keeps the pylon in place. If your pylon is like the one in the photo and does not have the plate, you'll have to drill the hole in the cup so that it line up with the one in the pylon.

Might be a moot point... Have you test fit it yet? That looks like a SN pylon and may not fit in your SW. In addition to the hull-floor distance being different, that pylon would have been installed in a cradled boat. Non-cradled boats like your SW had pylons with flanges at the floor. You're going to need to find a way to secure it, after you resolve the height difference. The pin at the keel is the least of your worries at this point I fear.

So I found the correct pylon for my 1984 Southwind at a Correct Craft dealer - Barry Capps in NC. It was leaning in the corner not installed in a boat.

It does not have a hole in the bottom pin that fits into the hull base of the boat. So I am wondering if I need to drill a hole through the bottom pin of the Pylon so I can run a clevis through and attach it to to the socket at the base.

Here a a few pictures of a similar hull base that I borrowed from another post with hole through it and someone else's pylon with a hole also. Looks like a match but I can't figure why my pylon did not have a hole drilled through from the factory.

Any help on this? What size is the clevis? Looks like 1/4 inch or so.