Re: Where does the ground/galvanic wire go?
This is my guess.
That wire grounds static electricity from the lower unit to the powerhead.
When I rebuilt my Johnson O/B (2 cycle) upper exhaust hsg. which included the tiller steering, I omitted connecting that wire. When I tested the motor on my aluminum boat, I heard a HUGE explosion at 3/4 throttle after about 1~2 minutes of running!! Scarred the crap out of me!!
As it turns out, I forgot some other things

and to correct, I had to tear everything apart and reassemble; this time connecting the grounding wire from the upper exh. hsg to the base of the powerhead. Never heard that explosion again.
Somebody with more experience may correct me, but I personally think it prevents a build-up of static electricity. If not grounded. the static elec. will build-up and eventually will ignite unburned gas in the exh. hsg.
Bottomline is to connect the upper exh. hsg. to the powerhead. The exact bolts that you use, to make the connection, shouldn't really matter.