There are some inexpensive 600v 400amp igvts on Ebay at the moment:
Mitsubishi Power x CM400DU 12F 1GBT IGBT Module 400A 600V | eBay
They did call them _Mitsubishi_ (I added underscores so it would cease to be a link) Power X instead of Powerex, which is a little disturbing. haha. But at least from the picture it appears to be the same as the 3 I have. He takes a "best offer". And all the Feedbacks he has (at least on the first page) dealing with that IGBT were for people who made an offer, and it was accepted. The Board is designed to plug straight into either a 600v 600amp IGBT or a 600v 400amp IGBT. The spacing of the gate and emitter spade connectors are the same for them.
I'm asking them if I can change it from 5 to 10 boards.