Shackle pin forward, allows connection point pin to pivot as the boat pitches.
Suggest a 3X strong shackle with a narrow 'throat' for the pin, ... if not build thickness so that as much of the pin's contact area is supported by full contact metal surface.
3X is the typical normal 'safety factor' applied to blue water boat design, I back-calculate that Bob Perry goes upwards to 4X in 'critical stuff'.
You dont want 'kinks' or bends in the 'tab end' of those plates .... good that the aft hull geometry 'tapers'.
With the pin forward, just a 'well made' bore hole will do .... such a 'slot' is a 'stress riser' which makes the functional stress bearing capacity of the plate less ... less than a simple well made 'hole'.
The hull sides werent designed for such loads .... think of being able to pick up the boat (stern up and bow down) solely by those plates and the bridle and not having something 'let go' or rip out.
I dont know what you mean by G10 ... you want a thick 'pad' (bearing surface) compressed between two plates .... the bolts forcing it all together in 'compression' so as to 'pre-stress' the connection AND the FRG between the plates (dont crush the FRG!!! ... calculate first!). The 'sandwich' is held together by the friction generated by that compressional force - a 'friction joint', the bolts are 'back up' if you lose the compression.
Without proper torquing of those bolts, and because of the differences in 'stretch' between fiberglass and SS ... the bolts will behave as a zipper in failure mode ... first bolt (from aft) will take almost take 'all' the strain and if over stressed in shear will rip the FRG, then the next bolt, then the next. Compression 'pre-stressing' tends to make the whole assembly work in 'unison' and over a much wider base / cross-sectional area of the hull surface.

As stated previously .... back calculate everything from the 'strongest' component of the bridle/drogue system ... multiply by 3 (Safety factor) when calculating the stern attachment stress cross sections, etc. When calculating to prevent failure of the pin mating section ..... use the 'projected' area (1/2 circumference) of the pin X pin length in contact on its 'saddle' ........ (1/2 of hole circumference X length of hole), design for YIELD failure, not UTS, and multiply then by 3 for Safety factor. Since the bridle can be fully loaded on one leg and not the other during a 'yaw', and all that stress then goes into ONE stern attachment ........ dont divide the max strength of the bridle by TWO for your 'starting point' of calcs.

May be moot, but so is ripping the top of the transom/'fanny' off at the wrong time, moot.