I remain skeptical about the whole premise that some guitars are better suited for fingerpicking than others. (For the record, I am a confirmed fingerpicker since the mid-1960s, and I own and play a slightly less antique and originally lower-priced Martin dread, a 1969 D-18.)

And I really can't see what the guitar neck and the left hand have to do with fingerpicking, which is normally conducted in the area of the guitar's body, not the neck, and with the right hand, not the left.

If you've really become so disenchanted with your beautiful vantage Martin, I'd prefer to see you sell it to a player who will appreciate it (not to a collector), take the sizable sum of money you should receive for it, and buy whatever instrument you'd prefer. I think that such a solution would probably be better for you ~ and for the guitar ~ than shaving the neck down.

Radically modifying the neck would undoubtedly hurt resale value. Of course, if doing so truly improved the instrument for your personal preferences and "fit," monetary value would be a moot point, anyway ~ you would no longer have any reason to sell it, and would presumably enjoy it for the rest of your life. (Your heirs might have a problem, but you won't be around to listen to their complaints...)

(My left index finger hurts, too, especially above the first knuckle, and my left pinkie is in even worse shape. Just part of the aging process, I suppose (arthritis, to be specific). Maybe I'm dull and unimaginative, but I can't see that playing a guitar with a narrower neck would make very much difference. Not playing at all might help, but for me that's not an option ~ at least, not yet.)