Here's a tip. I have a dead hard disk (the spindle is seized) that I used as dead load. You don't necessarily have to load the ATX power connector outputs, you only have to load the power supply so plugging a hard disk power cable into a hard disk (which when all said and done is low risk as you know that bit is wired up fine!!) and using that as load is a good dodge.
HOWEVER I've only ever modded AT and Mac PSUs which were hard-power, but I think the same rules apply for ATX.
I admire your caution. When I did my first AT -> A600 PSU mod i just wired it up, checked it looked about right and plugged it in
