Looking at SBD it looks like the intent is for the mast to pivot forward.
There are braces from the forward lower chainplates back and up to the cap shroud
where it meets another vertical brace from the caps grout chainplates. The cap shroud
turnbuckles are above where the braces meet. Those meeting points are in a line with the pivot for the mast.
You will need a bridle
that goes from at or near the clew end of the boom forward and out on each side to the top points where the braces meet. This will keep the boom centered under transverse load.
A. Furl the main & strike the jib
B. Attach the bridle
to boom and both cap shrouds.
C. Unpin the bottom of the aft lowers on each side, tieoff to cap shrouds.
D. Tighten up main boom topping lift
E. Tighten up main sheet to slack the backstay.
F. Pop out pin for backstay, secure backstay to a shroud or to the end of the boom. Probably there’s a best place to secure it but it’s not clear without having done this before.
G. Slack main sheet allowing mast to pivot forward, boom acts as gin pile. The stock pulpit has a dip in the center so the main is captured and held flat above the water
sticking out in front of the boat
some ways. Boom will be sticking straight up in the air.
H. If the boom sticks up higher than the clearance of the bridge you need to pass under, pop the bridle off on side which will allow the boom to flip over to the other side.
The assumption is you will right the mast immediately after passing under the bridge. If not then you will want to put a roller on the bow, unpin the mast from the tabernacle and slide the mast aft to a crutch at the back of the boat
Bruce Bingham has drawings of this in one of his books
. This one I believe: https://books.google.com/books/about...ok_description