I put mine on the centerline, about 1.5 inches up from the exhaust ramp rivet line (where the "roll" meets the firewall). Clear as mud?
I found this to be a pretty good location. Of course I don't use much cabin heat in the Houston area. The one thing that may be a negative is that the heat kind of goes up your pants legs. I guess you could put some sort of vanes or something on it to distribute the heat more.
Karl