If gimbal rings are shot and need to be replaced, you have (3)options:
a) pull the motors for access to the nut from the inside
b) the Mercruiser method, where you drill a hole on each side of the top for access to the nut. Merc sells a kit where by you drill a hole on each side of the housing, then tap it, for install of a plastic plug. Holes allow you to chisel nut at the top. the nut has serations just for this. This is a royal pain in the azz.
c) company from OH, sells a kit where by you drill a series of holes on the face, then punch it out for access to the nut. Once you punch it out, there is a stainless cover plate that goes over it. i've never done this, so can't comment one way or another.
As far as the external steering, yes it will go along way to prevent this from happening again. However, with the HP you have, it is not absolutely necessary. But, you will need to pay attention to keeping the nuts on the u-bolts torqued. Even if you elect to add the hydraulic steering, you still need to correct what you have.
The bad news: Those rings alone go for aprox. $700-$800ea. Don't forget to get new nuts for the top, they can get beat up taking them out.
While you have it apart, it's a good time to replace bellows, if they haven't been done.