Only back 2 header tubes glow

[Brent69]

I've read the posts regarding glowing headers and I checked the fuel mixture and timing and everything is fine there, but the header tube on each side of the engine nearest the firewall starts glowing red after just a few minutes running the engine.  The engine has always run hot and based on other posts on this forum, I suspected that it was the 440 Source water pump that I installed.  I changed that out to a Melling Hi Volume pump.  I was hoping to see lower temps, but did not.  It wasn't until last night while working on the car after dusk that I noticed the two glowing header tubes.

We checked the radiator and thermostat and those seem to be working fine - although the bottom of the radiator was really cool, almost like there is no flow of hot water coming in.  Temp readings with a laser temp reader showed low temps up at the front of the engine, but really high temps near the back and over 1000 degrees on the back two header tubes.

Here are the basic specs on the engine:
  '69 440 newly rebuilt with stock stage 1 ported heads
   Holley 4160 750 carb with electric choke
   TTI Headers
   Edelbrock Performer RPM Intake
   Aluminum Melling water pump
   440 Source aluminum water pump housing
   Stock radiator

Any thoughts on what would cause just the back two header tubes to glow?

[justcruisin]

Vacuum leak from the port on the rear of the manifold or at the valley pan from both sides at the rear due to ill fitment of the heads?

[Chryco Psycho]

or a vacuum leak through the brake booster