What timing mark do you use on a 98 vortec 5.7L?

You can't it is nonadjustable, The timing is controled by the computer. And there is no reason to adjust it unless you had the distributor out of the engine and in that case you will need to turn it by hand enough to get it running, and that is called the base timing. Then you will notice that the check engine light is on and you will need an OBD2 engine scanner hooked up to the truck and then go into the timing mode and ajust the timing to factory spec. and the scanner will let you know when you have the timing correct. The check engine light will go off when you have the timing right. Then the computer will do the rest.