You'll either need to be creative or have 2 people around, one on deck and one underneath. Try PB Blaster on the bolts before cranking too hard to undo them. Do not get any on the deck as it will stain.
If memory serves, you can follow the port water
tank vent hose as a feeler to get close as you'll be doing some of it blind. Be careful as the strands of FB were not sanded up inside the hull
there, what's left behind are are some pretty sharp edges. If you don't have it, get a universal joint for your socket. You'll need one anyway to tighten the nuts on your chainplates (which you should check and re-bed as they can work
lose over time).
Also, see MaineSail's article on re-bedding hardware
. You may/likely have core rot
under the stanchion base which should be addressed now while you have the base off. He sells the butyl tape too.
Re-Bedding Deck Hardware Photo Gallery by Compass Marine at pbase.com