[dsraven]

when thinking of an outside air pick up area for the heater, you could use a vent hole anywhere on the plenum that goes aross the front of the cab in front of the windshield. it comes out at the footwell vents and then drains below that, well, when I say drains it only drains if the drains aren't clogged at the bottom. as you likely have figured out, the drain hole is already partially blocked by the GM tin flap they use to partially cover the drain. dunno why they did that but they musta had their reasons i guess. anyway, it doesn't take much pine needles or leaves to build up in the small drain hole and then it is almost fully blocked, when that happens the water will build up in the plenum until it reaches the kick panel vents at which time it will run out on the floor of the truck and then down to the steps, especially on a heavy rain or when the snow is melting fast and the drain is iced up or plugged. this eventually rts the floor, hingle pillar, step area and anything below that, like the cab mounts. on my cab the floor seemed to go downhill towards the front and that was because the weight of the front of the cab was sitting on rusted out cab mounts and hinge pillars so the floor was bending under the weight. thats one reason why I used the floor and firewall from my donor vehicle when I did the frame swap.