Well, don't overengineer this pretty simple task!
Mount the winch in a convenient spot beside the companionway
. Put a backing plate on the inside to spread the load. IIRC, I used some 1/4 " aluminum
that I had around.j Be sure to seal the bolt holes well with either butyl tape or some form of sealant
like Sika 291 or Lifecaulk. I don't recall
if the cabin top is cored in that area (or at all... I sold
that boat in 1978), but if it is, read up on how to remove some core
around the bolt holes and replace with epoxy
and West 403 or such. This is to keep the bolt clamping force from crushing the core
and deforming the GRP.
Again IIRC I mounted the turning block to the cabin top using a big saddle and a backing plate inside. The block was one with a half ton or so SWL, and that is overkill IMO. You could mount the block to the mast, but I think it is harder to get a suitably strong attachment there. Some sort of hound like for a dinghy
forestay would do... Ronstan has some such in their catalog, as do other suppliers.
I've probably got some photos back in California
... they might as well be on the moon!
Good luck with your project
. It was quite worthwhile for us.