Im just about done with a similar install. In my case I had an anchor locker
bulkhead to mount a proper chainplate on, 3/8 SS plate with 5 hex bolts 1/2" and a G-10 backingplate on the other side of the bulkhead. Im making my stay from either covered vectran (3/8" crystalene) or possibly 3/8 Dynex Dux. I subscribe to the rule
of keeping the top fitting within 6% of mast
height for no runners so my top fitting is going to be about 2.5' down from mast head
(51' spar). Im going to use a T-terminal with backing plate. There is no need to take the rig down for this install. One simply cuts the appropriate size slot (rounded ends) and use a piece of stiff wire to hold the backing plate in place while riveting. Its a lot easier to do with the mast down but not a deal stopper. So my fiber stay will have a thimble at the bottom where it will attach to the highfield lever. The top will have a T terminal with a ring that the other end of the stay will be spliced to. Just below the T terminal will be a sheave box for a halyard
. This way when we are not on passage
I can send my boy aloft to take the whole stay down.