If you run the hear into any part of the chassis the metal mass quickly reduces the air temp. I have tried a few way including direct into the center tunnel via the rear wishbones ( have to weld in a thick wall pipe as a round gusset) Problem was the shifter and E Brake hot really hot to touch. The other way is from the heater box directly to the base of rearward vertical door jamb..this makes for some serious heat but as mentioned engine noise. One way to reduce the noise is to make up a piece of plastic pipe with alternation half moon shaped pieces in the pipe this makes the heated air travel in a lazy S' path and helps to reduce the engine noise..