Hmmm... That's not going to be easy unless you have an 8100/80 to look at. The multiplier on the 100 and 110 MHz machines is three, so if you change the oscillator to 40 MHz, the CPU will run at 120 MHz unless you can change the multiplier to 2.

On the other hand, when you install the NewerTech, the 601 should idle, so the speed shouldn't be a big factor.