The stock intake manifold doesn't allow exhaust gas in, it is heated by the exhaust though. I'm pretty sure that putting tubes from the headers to the bottom of the intake will not do a thing. The stock exhaust and intake are connected of course, so heat transfer is instant. If you run the exhaust through a couple tubes for a foot, the heat will disipate, know what I mean? By the time the exhaust warms the intake, the motor will be warm. That's my theory. BTW, I had an 81 C20 that I put headers on (350)... didn't worry about a heat riser pipe. How many custom vehicles have you seen with an exhaust heat riser?
