I'd agree with the why cut holes in a perfectly good boat, if it were new, but it's not uncommon at all with the 70's boats for the bedding material (the "donut") to fall apart and leave you with a broken mast step and/or destroyed deck around the mast.
Like Al said, cheap insurance. - Check it, if the bedding material is solid, add a layer or two of mat/resin around out and rest easy...
If it's not leaking I wouldn't touch it. Pop a disc in as said earlier. Put the lower section in and push on it and see if the deck moves around the top of the mast hole. If not, it must be fine. Otherwise you could put the inspection port on the flat panel next to the board case and look forward.
I had a 76 boat that I did this with. You will NOT be able to do a decent glass job if you are trying to reach from an inspection port located by the dagger board trunk. You will need to locate it on the upper deck near the mast step. I put mine directly in front of the step about 18"-24" away from the step, but I have since learned it is best to locate the port at an angle in front of the step. Either the port or starboard side/forward quadrant at about a 45 degree angle from the step. An ounce of prevention is worth a pound of cure....................
I'd put it 10-12" from the mast. This makes it easier to work one armed in the hole, sorta the distance from my elbow to my hand. If you put it astern, and offset from centerline a bit, then you can reach other fittings that may need some work, or a nut on the back.
Beside the DB trunk doesn't reach much, and bangs the knee a lot, assuming you sail downwind at max forward in the light to med stuff. I suspect Vanguard stopped putting ports in at the factory, because placement choice ywas so controversial, and not really because the might leak, etc. They were a standard part of a new boat ( on the right of the DB slot) for 6-7 years, or so.
You can do other external tests before cutting if you want. Weigh the boat. Anything over 130 lbs is water. And measure the depth of the mast pocket. Anything over 14" is wear, and over 14.25 inches is worn thru the pocket.
Do some searches on those topics for a full description.
its not to worn the the little nob thing is still there and im not all about puting the plate on the deck around there because to me its ugly and it would be hard to get to because i want one with a bag. after every sail i do a drain so no water