Setting timing on a Vortec

I recently had the engine in my Tahoe rebuilt, after reinstalling it, I am getting code 1354, which is a descrepency between the CPS and the distributor. I have been told that the computer corrects all the timing issues, but also have been told that I need to adjust the PCM with a code reader. I have a Snap-On tool, but as yet havn't been able to find the information on how to do it. Any ideas?