I mounted mine beside the passenger side headlight between the radiator and grill on a 72 Mach1. I know it sticks out like a sore thumb but the wiring was so easy. If you drive in rain at all I would not recommend this spot because it could get wet. Other areas I considered were behind the air cleaner on the firewall, on a shock tower, or underneith the washer fluid bottle. I have heard of people mounting them inside the car too. Cool, dry and as little vibration as possible is what you want. Make sure it can breath too, do not put it in a enclosed space.
How about the top of the hood! That'll keep it cool! Drill a nice big hole to run the wires through, bondo a scoop around it, kind of in the "Hood tach" position. Come on, you know I'm just kidding around!
I made a bracket that mounts between the hood latch and the top of the radiator support. The box is mounted behind the hood latch and hangs in front of the radiator, it is nearly invisible. I have the template if you want to use it, I can fax it to you.