Ah yes.   Our boat is the same vintage, and a few years ago, when I replaced the life lines, I decided to upgrade the aft-most stanchions to true gate stanchions  (like those next forward), since they get a lot of stress  (which may be why you have leaks) and are easily bent.  Catalina Direct sells them, see photo.   Anyway, I digress.  
       Yes, getting access to the underside of that stanchion is a total PITA; I agree that Catalina must have done these while the deck was apart -- I can't see how it would have been done otherwise.     One of the worst access issues I've found in over a decade of ownership, alas.    
        The only way I was able to get access to the general area was to cut some holes in the outboard side of the port locker.  But even then, after emptying out the locker and crawling into it, and snaking my arm in there (not for the claustrophobic) there was no way to reach the underside easily to rebolt.
       This is one location where Catalina should have put in a backing plate with threaded holes, which is what I ultimately did. 
        I made up 2 backing plates (since the replacement stanchion has 2 feet) out of 1/4 aluminum plate, into which I driilled and tapped the appropriate holes to match the feet of the stanchion.   If I recall correctly (this was a few years back) I then applied slow-setting epoxy to them and slid them into position (probably tacked onto a stick) while from above I screwed the stanchion down with hex bolts (the original Phillips screws will never enable you to crank it down as needed). Once the epoxy set, I was able to unbolt the stanchion, apply butyl tape as a bedding compound, and reinstall.    
   
   

To access that aft port stanchion, there should be 1 or 2 4" or 5" access holes in the back wall of the cockpit locker. Mine had two next to each other with sharp points where they intersected. Perhaps a PO cut them in. If not there, buy a 4" or 5" hole saw bit and cut in two holes just aft and below the stanchion base where the bolts would come through. Lie in the locker and bump your head a lot and use a 7/16" wrench to loosen the nuts. Of course the LAST nut and hardest to reach was seized so I had to drill out the bolt from above. I had 3 stanchions with hairline cracks at the base, so I bought new ones from Garhauer Marine. I had found a used gate and then bought a single stanchion for $65 and the gate was $105 and then because of a manufacturing mistake, I had to buy another for the aft PORT side, which came with carriage bolt to be used in the base. At first I was just going to use the regular bolts, but then realized in that small space, I couldn't clamp vise grips in there so I ended using carriage bolts. Works great in that spot. Just insert them and then you can tighten bolts from under side through the access hole and don't need a helper to hold the top of the bolts. I drilled out the holes and filled them with West System 105 and then redrilled the 1/4" holes. The old holes were sloppy. I used 3m 4200 to bed the stanchions- messy but cleans up with enough effort and doesn't attract dirt like the butyl tape. I also put the washer on first, then the lock washer, then the nut. Hope that was right because what I took off was lock washer, washer, nut. Seemed backwards to me. Garhauer has great prices on all the hardware on our boats including the blocks and chain plates.

Do the bases of the Garhauer stanchions match the base of the Catalina stanchions that are original?

David, Most likely, the Garhauer Marine stanchions ARE the original factory stanchions. The standard stanchion base for our year boats is the ST58 with flat top. The ST52 in the photo I added to my earlier post, is the beefed up version with thicker tubing and raised welded bases... but also has square holes for carriage bolts (or I guess you could use round bolts into square holes). That one works well with carriage bolts for the Port stanchion that you can not easily reach to put on vise grips if working solo. I replaced my cracked stanchion with new aft gates, Port and Starboard. I used regular parallel gate on Starboard side and beefed up version on Port side and other than the carriage bolt head vs round phillips head, and the slight beefier raised weld, you can not tell them apart sight or touch. Garhauer is very freindly and easy to work with, just call Mike at 909-985-9993 or email info@garhauermarine.com(link sends e-mail) and tell them what you want. They can mix and match bases, tops, etc. Also, that photo of the gate in post #2 with the angled brace leg is more money at Garhauer than the one with parallel leg. I used standard tubing gate with parallel leg so it didn't stick so far into stepping deck area and it was only $105. I think angled is $130 or 160?  So my forward gates are angled and aft gates are parallel.

For anyone wanting to avoid the expense and PITA of replacing the aft gate stanchions, I came up with this cheaper, although not foolproof (still can rock side to side a bit, but not much).  Two open-jawed 1" fittings on each side and two lengths of SS pipe.  I removed the aft section of lifeline and simply put a SS nuts and lock washers to hold the back of the short gate-piece of life line, capped with a SS cap nut. 

I just put the True Rear Gates on myself, its a pain to do the bort side, You basically have to get in the locker and reach your arm into a 5" round hole that is back there, your doing it all by feel, be prepared to lose lots of nuts/washers as you will drop quite a few into the depths of the boat.

Wouldn't it be easier to go through the back of the galley cabinet above the fridge?  A bit sad to cut an access hole on the teak, but it also seems to be a short reach.