Gm van

1987 GM 3/4 Ton Van, 350 cu inch engine. Rebulit 5000 miles ago. New radiatior, hose, water pump, thermostat. Timing checked. New transmission.

Engine started overheating. No loss of water/antifreeze. Temp 235 degrees front of engine, 285 degrees rear.

Overheating started on day outside temp was in 20s. Small knock developed.

What am I missing!!