On page 263 is the following statement: "The flanges were bolted through the deck, with nuts accessible through the lazarette. The gallows assembly needs to be very strong." In my case, even though I drilled the holes in the exact center of the seat-back tops, the poles hit the sides of the hull before reaching the deck. So I had to build it up with bits of 2x4 glued to the deck to get the flanges high enough to meet the poles. Rather than try to drill holes in that very small space, I decided to put some pretty hefty wood screws into the 2x4s instead. (If the flanges were sitting down on the deck, bolts would probably have been a better idea.)
I don't think I will have any problems with the screws instead of bolts, but the "gallows assembly needs to be very strong" part leaves me a little confused. Bolting the flanges could imply that there will be a strong upward pull on the flanges--the wood screws will easily handle any side-to-side forces and quite a lot of upward pull. And, the screws will most definitely withstand a lot more upward force than the little set-screws in the flanges will. Then there is the attachment of the gallows itself to the poles to think about. At this point, my gallows is just sitting on the poles. It is a very tight fit and requires some force to remove it, but not as much as the flange screws (or most certainly bolts) can withstand. So am I supposed to put a bolt through the gallows and poles at the attachment point? That doesn't make much sense to me with as I said nothing but the little set-screws holding the poles in the flanges.
Also on page 263 is a statement that holes are drilled on both sides of the gallows notch to tie down the mast and boom, etc. while on the road. To me this means that road travel can cause the rig to bounce out of the notch. I don't see that creating enough force to pull the poles and flanges out of the deck! At the worst it might pull the gallows off the poles. That does not seem very likely, knowing how hard it is to get the gallows off the poles, on my boat at least.
I think a better solution would be to run a line around the rig and attach it to the two stern cleats. (And try to avoid rough roads! Bounces that are hard enough to pull the gallows apart would do much more damage in other places it seems to me.)