Ironhand,
In order to answer your question, I can say that the hot bluing process we used on our double rifles some years ago did not seem to bother any of the seams on the barrels and both the 4 bore and the 8 bore rifles stayed together with 300 grain Fg charges.
You should not have any problems putting them into a tank.
Sorry to say, I do not recall what all the brand name of the hot blue solution was, but it was obtained from Brownell's. I don't think they have many choices to choose from.
On the other hand, if it is more than you want to deal with, I am sure most any gun shop that gets into repairs could handle the job for you. Probably less money than investing in your own hot blue set up, tanks and such.
Another thought is to go ahead and do a cold brown finish then heat that up. It will go from brown to blue.
John