The builders of our boat(I have#66) have sunk 1/4" steel or iron plates behind all of the bolts for not only your stanchions but anything that appears to be thru bolted. These plates are drilled and tapped to accomodate the bolts. I used heat, manual impact driver with a perfect fitting slotted bit, penetrating oil and a ton of patience. It is a challenge to get these out. When I go to replace the bolts, I will likely move the bases forward by 1/2 -3/4" and just redrill and tap into clean steel......I don't think that moving everything forward by this amound will make any difference in the big picture. I have basically removed everything from the decks for repainting. Teak decks are gone too!!!
S/V Iemanja on Lake Champlain (for now..)