You have the VIN code "K" 5.7L in your GMC. (TBI engine)
Was the vehicle "Pre-Conditioned" before you checked the timing? (coolant temp. above 160 degrees, Idling in park, with the EST (electronic spark timing) bypass wire disconnected)
The ignition timing on the K motor is "0" degrees TDC!
NOTE: The EST bypass wire (single connector, tan/black wire). 1986-93 connector is in engine compartment. 1994-95 connector is under dash below glove compartment.
The "K" 5.7L is a "Straight up" (0 degree TDC engine)