We have a B38 Lagoon from, I guess ,the same era...
They didn't use nuts on the underside, but instead they used multiple small individual S/S plates about 40mm / 20mm / 3mm (approx.) that are threaded at one end to take the countersunk coach bolts......
(Why didn't they drill and tap them in the middle..?? I've no idea..)...
Access involves removing whatever headlining or carpentry that's installed (on your particular boat) on the underside of the Genny Track...
It'll take you a while to access them all, and a fair bit of dismantling and reassembling....
Also, due to the sealant that they used some, if not all, will be very difficult to remove...
Best to gain full access to the underside first, and then attempt to undo and remove the S/S plates from below and then punch the coach bolts up and out...
It's a job I should also consider, but having had cause to replace a few and suffered the frustration, I think until it leaks I'll leave well alone....Bill.
Good luck..