I remember making the conversion from scratch on my 4000 long ago but it was an AT power supply, not ATX. Never the less I used a voltmeter and checked every single wire BEFORE plugging anything.
Make sure you get the correct voltage on each prior to connect, that's the best advice I can give.
I checket the current on my present powersupply.
It gives the following stable readings:
5V=5,17 volts
12V=12,21 volts
-12C=--11,45 volts
I am not sure if theese are "safe" numbers.
Any comments?