Inline 6 cylinder motor mount adjustment

[moneyhole]

I have a 77 V-20 with a 165 hp Merc inline 6 banger and it has become apparent the engine is very slightly out of alignment. It was running and shifting, but when installing a newer stern drive, I couldn't get the input splines to line up and I am looking at renting or welding a tool for such.

Here's my problem, how on God's green earth, or in our world blue-green, do you attempt to adjust the motor mount shaft - up/down. There is simply no room to get large wrenches into the engine box which is closed in from the factory where the adjustment shaft is. I don't see how it can be done. My engine only needs about an 1/8" adjustment (guessing without the factory tool )but what do you do to loosen the large nuts, adjust your shaft up or down , then tighten the large lock nuts ? Is there a special tool or do I have to cut out the enclosed engine box (don't want to do this unless absolutely necessary)
Open to all suggestions. Thanks