420 MHz? As far as I know the fastest ever 604e is 375 MHz native clock that can be overclocked to 400. Then 420 would be the something over my expectation.
I have also CSPPC upgraded by Stachu, but I mine has 604e with 350 MHz native clocke and underclocked to 333 MHz because I wanted to keep my 060 rev6 by 50 MHz. I have to say both CPUs are much cooler comparing to 060 rev1 and previous 604e/200.
Anyway please share with us your Syspeed module then ;o)