1990 GMC Sierra Won't Start after Plug, Wire, Cap & Rot

The truck was running okay, with some hesitation and occasional back fire through exhaust when accelerating from a stop. Parked truck, changed plugs (.035 gap), wires, cap, rotor. Truck would crank, but not start. It appears to be a timing issue. The distributor is tight and was not moved. We have verified firing order comparing to numbers in the book and on the intake manifold. Checked that the #1 cylinder was at TDC and the rotor was pointing at the #1 wire. We have spark at the plugs. We have fuel spray from the throttle body injector into the manifold. Rotating the plug wires one position either clockwise or ccw caused the motor to backfire through either the intake or the exhaust.