hard to say for your setup... on my P26 I had to construct a hump out of teak that epoxied & screwed into the top of the tiller to give me the mounting point at the right height for the pin.
The body of the ST2000 & the pin on th eother end rest on top of the seat (not coaming), where I drilled a hole through for the brass cup, which is set into a 4" x 4" x 2" block that I expoxied to the underside of the seat (so no flexing at the "donut" end to crack FG as you describe). You do want it to be a level mount for the tillerpilot.
Good luck, it took me (and a friend who is very handy) some time with measurements & angles to figure out how to line everything up as best we could.
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Pearson 26 #1200