How can I set the timing on a 1996 GMC V8 5.7l?

I don't think the timing can be adjusted on that vehicle...it is supposed to be controlled by the ecm

Make sure you put the rotor on correctly. It should only go on one way, but I have seen one forced on 180 degrees out, mine had one of the alignment pins broken off.

You cannot. If you think you are having a timing issue, you can check by removing Cyl #1 spark plug, then, while keeping your thumb covering the #1 Cyl spark plug hole, rotate the crankshaft to TDC on the compression stroke. Pull the Distributor cap, the rotor should be pointing to a small > at about 8 oclock on the edge of the distributor. There should be a small 8 in the > as you have a V8. I hope that helps.

timing is only computer controlled on this vehicle it does however i believe have a connector under hood left side of firewall tan in color or tan and white if you disconnect this connector you can manually set timing to zero and then turn off and reconnect wire start vehicle back up and it should automatically reset timming to factory default

Like jimchevy said, not adjustable but it also can't be out of adjustment so don't worry.

I have same problem on 1997 GMC 5.7 K2500 207000 miles. Found out Distributor has a soft gear on end of shaft that wears to a knife edge. This throghs timming into a state the ECM cannot figure out so it starts stummbling and then backfies resets and slowly starts to stumble again repeating misfire. replaced with new ditrubutor setting base timing by listed procedures. still getting an ocasional timing code. Now find timing chain has to much slack throwing it off have to replace it next. hope this will help someone.