1/4" threaded rod is fairly inexpensive in 4-foot+ sections, and is one of the best ways to connect 2x4 studs together after drilling a hole right through both (or multiple) pieces. A larger box of 1/4-20 nuts and large
fender washers nets a really strong connection and will not loosen-up like long screws -especially long drywall screws which are brittle and can be easily broken/snapped on a shifting and flexing boat deck, and can split most lumber without pre-drilling the holes anyhow. You can buy a longer 1/4" drill bit that is about 10-12" long which makes quick work of this kind of job. A c-clamp helps locate cross-beams before drilling and making a fastener with a piece of threaded rod. Cut with a hacksaw and clean up the threads with a file.
Resist the urge to use the bow pulpit and/or stern real as part of your framing. On most sailboats these structures are just not up to the task of holding the heavy spars unless the
boatbuilder had some sort of plan in mind when the boat was constructed for hauling the mast on the boat when unstepped. I've seen a few really nice factory mast supports on C&C's that were slick as heck and built out of
aluminum that snap right into the rails and the mast step.
When building something don't forget about the boom(s) as well. Leave space on your cradle for these too, as you don't want them on deck and in the way underway.
I like using Ancra aircraft-grade loading straps. I used to
race motorcycles and have a bunch of them around still and they work really well for this kind of load Don't just go buy cheesy ratchet straps that are not secure or strong enough. You don't want to lose your
rigging overboard when you get waked really badly by some clueless powerboater. Without the mast up a sailboat actually rocks even worse than it does when it is up which is a bit counter-intuitive because of the pendulum effect which slows down roll. Be prepared for the worst-case scenarios on a long trip like this.
This is the last time our boat had the mast down and the cradles I built. It was only for a short trip down the Chicago River and a few days sitting on a
mooring field in a notoriously poorly-protected Chicago harbor.