I had this problem also with my 63 K-Sub- replaced the door weatherstrips and voila!, the doors were impossible to close.
Another poster on this subject eluded to some "hot summer days" that will help these seals to conform over time. I live in AZ, and it gets plenty hot here and still had a problem with being able to close the doors fully.
I wound up ordering a pair of their cab door seals (I did not use the top seal on the door- I don't think the 62-66 use this as the 60 and 61 apparently do).
http://www.soffseal.com/store/main.a...tail&item=9008
(cab seal), they also have the top of door seal, item = 9972
When these (softer) seals were put on the doors, HOLY COW, what a difference! Both doors close effortlessly and seal better than how they came from the factory. I am pretty confident that this would solve your problems. My doors close without any slamming or extra effort on my part, and these seals keep the elements out- no wind noise or leaks. I am a happy camper since I installed them, been about 4 years ago now.
Give it a go- you won't be sorry. About 75 bucks for the main cab seals pair, and another 45 if you want the top of door seals.