Depending on the time at temperature, you should also be aware of the possibility of sigma in conjunction with sensitization. Naturally, a stabilized filler metal such as a 347 will prolong the onset of both. Maybe cladding after the hot forming can be considered. Like you said, a 3.25" thick metal will remain in the sigma range if the time is adequate. If this is held for around 4 hours, it is likely sigma will form, and almost definitely sensitization.