If you never used one before, here are a couple of tips:
A light hammer and a sharp punch will usually give you enough of a indent for the weld cutter to ride in until you get a good circular channel started.
But not always, so keep another drill handy with a bit in it that is just a tad larger than the spindle on the weld cutter. This will give your weld cutter the stability over uneven spots to get the job done.
On the back of the cab be very cautious when cutting else you will be plugging holes you didnt need to (I tried using an air chisel and learned the very hard way).
Remember the spot weld cutter is sharp, and fast, if you are not paying 100% attention you will cut dime size holes clean through both layers of steel. In my tiral and error learning I found that it was better to under cut it, and tap tap tap with a chisel between the two welded pieces, than it was to cut all the way thru.
I hope this helps you (hopefully you read this before you get hands on the project) and good luck !!
Here is the build I was looking for, a great fellow doing an awesome build:
http://67-72chevytrucks.com/vboard/s...=513377&page=6