350 engine backfires through intake

My son owns a 1998 Chevy Silverado 4x4 with a 350 A/c powersteering, etc.

The other day it would not start and acts as if it is "loading up" a cylinder with fuel and back firing through the intake.

At a Chevy Dealers direction we replaced the Fuel Pressure Regulator with no change in results. We checked the timing and it was correct, but even so we pulled off the front of the engine and the timing gears and chain were all good. Believing it may be a rounded lobe on the cam we turned the engine over with the valve covers removed and all the push rods are making a full travel.

Any help or suggestions would greatly be appreciated.