If the center of that raised boss is above the gear when the PTO is engaged, I'd be strongly tempted to drill and tap it for a pipe plug, either a 1/4" NPT or a 3/8" NPT. Then you could monitor the tooth engagement and adjust the shim pack accordingly.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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