Welcome to the forum. It is geared more towards "big trucks" but everybody interested in conversions is welcome, and your issue is common with truck builders as well.
There was a thread a while back dealing with cab seals which you may be able to find, I can't remember which it was, but this website was listed as a source for accordion seals:
Uni-Grip Sealing Products
The poster said they are very helpful in finding the right product for you if you shoot them an email.
I remember the walk throughs they were building back in the minitruck days, and most of those were done by welding the cab to the bed. They made matching cutouts in both, and formed "u" shaped sheet metal pieces to weld in and box in the gap on the bottom and sides. Then they uses one of those fat foam cap seals to seal the cap to the top of the cab. Welding it all together on a mini-truck did not seem like a bad idea, basically just made it into a mini-suv. On a one ton you may want to think about flex if you are hauling any weight before going that route, or just making sure the joint is beefy enough to handle it. The same issues have been discussed on here for truck conversions about welding it solid vs. a rubber boot. Opinions vary.