There is a technical article on the ford vintage. VFF.
The link I had provided does not work, so I have removed it.
This is the most informative and easy to follow article that I have seen, concerning the adjustment of the cab and doors on the A and AA truck cabs.
Although it is written specifically for the 28-29 82-A cab, it is generally the same procedure to follow for the 30-31 82-B cab.
I had a very similar problem with the doors on my truck. What I found was, that over time, the body block rubber pads had continually compressed to the point where the doors would no longer latch, and the doors appeared to be 'sagging' at the latch side.
Following these instructions, and replacing the rubber pads, my doors now fit properly, and latch as they should.
When replacing the rubber pads, you are best off to purchase some 1/8" thick reinforced rubber belting from a Tractor Supply, or similar store, and cut your own. The rubber pads sold by the vendors are made of plain rubber, and compress easily when the body bolts are tightened. The only thing the vendor supplied pads are good for is to use as patterns, when cutting new ones from the reinforced rubber, which is what I did.
The ford vintage, along with AA Fords.com, are sites with a wealth of information, and invaluable articles, as is Ford Barn.
Good luck !!