You will want to degrease it thoroughly as the preservative (wd40) is in all the nooks and crannies as well as the pores of the cast iron. I would use lacquer thinner and some rags and maybe a brush to start, scrubbing the block. Then I would probably move to a dewaxer like you would use to prep a car body for paint. I think doing this a few times should remove it.

I paint mine just like you would paint a car body. Epoxy primer first, then whatever color paint you like. I've used Urethane and most recently Imron as I like the way it flows and shoots. Never used POR15 though.

I would also address some of the work that was done 20 years ago, like if you had gaskets, seals, rings, etc installed. Those are probably brittle by now, but it sounds like you just stored a machined bare block from your post.

Don