Hi DockRate- I do not have the templates around, nor do I have the boat anymore to be able to measure them. But if I recall correctly, the chainplates were about 10 inches tall and 4 inches wide. I had to put all new rigging on the boat and new spreaders, so the rigging length changed. Since I had no way of calculating this I used screw-on lower wire terminals (Hayn Hi-Mods). For what it is worth, I believe that the horizontal straps that the chainplates are welded too are probably sound. The problem seems to be in the area of the welds or where the chainplate passes through the deck. When I did this job, I added big 1/2" G10 backing plates over the remains of the cutoff chainplate, and glassed that over to the hull side. Probably the G10 alone would have done the job.
To see if your shrouds will reach the new chainplate locations (I assume the mast is upright on the boat and I assume that the chainplates will be moved to the exterior of the boat), I would first cut away some of the fiberglass that overlies the existing chainplates so you can locate the position of the horizontal strap, since you will want to tie into it. Then mock up a chainplate out of wood or cardboard. With the mast properly supported with a halyard, remove one of the shrouds from the existing chainplate (mark the threads with electrical tape first!), and see if there is enough room for it to reach the new outboard chainplate position. Then replace the shroud and tension back to your tape mark. Repeat for all six shrouds. This should give you a reasonable estimation of how tall your chainplates must be. Keep in mind the shorter the better, to eliminate flex. As for width, I made mine wide enough so that I could get one bolt through the horizontal strap on each side of the remains of the welded upright chainplate
Also in your design, make sure you have enough meat between the edges of the chainplate and the hole for the rigging pins for the loads expected (there is a formula for this somewhere).
I'll dig around for some photos.