Jadis JA 50s power tubes

I contacted Jadis in France regarding the possibility of using KT-120 power tubes for my JA 50s. I received a response from Patrick Calmettes saying only KT-90s should be used in the amps due to high voltages. However, my amps have been using EH KT-88s, which they were supplied with upon purchasing them used from a very knowledgeable Jadis technician in NY. The amps are self-biasing....anyone have any experience using these amps?
Nobody has any experience with JA 50 amps?
Even if it was ok to use, are you sure they fit? KT 120 tubes a little wider than KT 88 that you currently are using, right?
I'm accepting the info from Jadis that KT 120s can't be used....not sure if the physical size would be a problem. What I am questioning is the insistence on KT 90s, since I've been using KT88s. I want to try Gold Lion KT88s and see how they compare with the EHs.