The question was can you get something in the seams to remove the rust and leave the paint ...
I would use ospho to deal with the rusty seams buti wouldn't count on the paint being the same afterwards. (See my many warnings about proper neutralization of ospho before using that route)
My experience is that it will.creep under any loose paint and either lift or hide... the first you may see right away the second could bite you after you think your all done.
It's a question of what you want the truck to be and how much effort your willing to put in vrs the risk of paint failure down the road. If its gonna be a work truck who cares about the rusty seams. On the other hand if your looking to preserve it how long do you want your efforts to last?
Once you start down the road of removing rust it becomes a question of how are you going to protect the metal and do you trust the 50 or 60 year old paint to stay stuck?
I don't have that much faith.