I once worked in a factory where we made a similar sized beast.
What we had to do, for long term reliability, is to over-plate the PWB copper layers to 4 ounces.
This to collect the current from all the transistors into a centralized area. From there, a buss bar was used to take the total current to the output connector.
Edit; I hadn't read your last sentence, 100 Khz operating frequency. Do you mean that you are going to output 100kHz at 250 amps?
Then you will have to check for skin effects, and determine the effective conductive cross-section?