Very high idle speed

I just installed a new Road Demon Jr. 625 cfm carburetor on my 350 V8. When I start the car, the idle speed runs at about 2500 RPM until the engine warms up and the electric choke releases. Then, the idle speed drops to around 2200 RPM. I tried adjusting the float levels and idle screw on the carb with no luck. I then tried replacing all the vacuum hoses and re-routing them to match my GM service manual depiction for a V8 with a 4 barrel carb. While standing by the car wondering what could be the cause of my problem, I noticed that the exhaust pipes for the #7 and #8 cylinders were glowing orange! My guess is that that the motor is pulling air thru a leak in the rear seal of the intake manifold, creating a hotter burn than the other cylinders, and driving up the RPM level. Does anyone have any recommendations for me to try?

sounds like you allready got it figured out but to check for intake leaks or to verify just start it and let it idle and mist top of motor with starting fluid , if there is a leak motor will rev up when the ether hits the leaking area, be carefull when doing this though due to flamable properties of ether, keep arm extended and face back in case it ignites, if it is the intake which sounds likely just replace the intake gasket,also if you just put that carb on and had no trouble before you may have wrong gasket or a bad gasket between carb and intake.