have you tried probing the PSUs voltage rails with a DMM to see if your getting any voltages from it? that would help rule out if its a PSU issue or not, I have never actually looked at what sort of PSU the 6400-6500 uses, wonder if you could hack up an ATX PSU and get it to power the machine...
There is an old article on a French website that details how to use an ATX on a Performa but it involves cutting the case and looks like a fair bit of work.