1) did you drop your keel before installing new bolts, or did you replace them one at a time with the keel in place? In place, the driver for this decision was an excellent keel/hull joint with no weeping, movement, or apparent damage to the joint or adhesive
2) If in place, did you replace on land? Yes, on land, and I would think it is worth the cost of a haul out
as extra assurance for the integrity of the keel/hull joint. Doing them one by each in the water it's possible the joint could open a bit and be compromised, blocked on land it can't.
3) Was it a deep draft fin keel or shoal draft? Deep, 8'
4) Did you need to open up part of the interior
floor boards in order to get to all of the bolts? No, but had to remove and replace the hot water heater to get at one or two of them. The 405 wil be different, of course. If you call beneteau they can tell you how many bolts there are, and you can eyeball their locations to get some estimate of the difficulty. We had 13 bolts, and we saw and counted 12 before we got the number from Beneteau. The 13th we didn't see (it was the one under the hot water tank, obscured by the tank, wiring
We got the bolts and washers from Beneteau USA, they had them in stock and reasonably priced. Our only departure from the original install was to have the bolt heads brushed with clear epoxy after install to preserve them long term. Two years later, they appear the same as the day they were installed. The work was about 3 days of yard labour, using 1-2 workers, and none of the bolts snapped off or was too hard to get out. If one of the bolts had snapped or needed extra persuasion, it would have been ugly, labour-wise. You do need a huge socket wrench, piece of pipe to extend the wrench handle, and a massive torque wrench to do the job. As I think I mentioned, the parts
of the bolts we couldn't see before taking them out were just fine. They appeared as new.
On the 405, isn't the engine over at least some of the bolts? You may need to lift
it to get at them, and that would lead to higher labour costs.