The coupling bolts are on the prop shaft coupling. Follow the shaft up out of the stuffing box until you hit a flanged joint.
It will have at least four bolts going through it.
The motor mounts being loose may mean you are out of alignment. Google engine alignment for inboards.
It is easy to check, and is done right at the coupling I mentioned.
Think about it. The v-drive is what the prop pushes against to move the boat. It has a bearing to absorb the thrust and transfer the work of the prop to the hull.
The v-drive is bolted to the trans and the engine. If the engine isn't bolted down tight, the prop will push it. If it pushes it at an angle (turning hard) it will vibrate. Badly.
Checking alignment will require a couple of wrenches and a feeler gauge. If the mounts were loose, it is probably off.
I am happy to report my cavitation/vibration issues are gone!! I got a new ACME 1433 prop and the issue went away... Evidentially the company that refurbished my prop didn't do so well. The 1433 prop is AWESOME! It pulls great and the hole shot is amazing. Thanks to all for your input.
I went through that a couple years ago. I had some other problems too but also had a poorly repaired prop. I sent it back to the outfit that repaired it and made them get it right and include a report on the balancing. They did get it right the second time. Scared the crap out of me though.