I sanding to the metal, takeing all the old layers of paint off. I've seen Blue primer or sealer that looks factory. Should I take this off also or leave it.

I'd take it down to bare medal. If it is original primer, I would want it in little piles of dust on the floor, I'd want to start from bare medal and use the etching & filler primers that are much better today than the primers were 30+ years ago. When I striped my 71 & 68 there was no sign of the original primer when I was finished.