I mounted mine right on the firewall behind the distributor. It ran that way for years.

Here are two points about pointless ignition:
1. Keep the signal cable short. That's the cable from the distributor pickup. The longer the cable, the greater the chance of EMI.
2. Keep the signal cable twisted. That helps the linterference to cancel out. When I ran my box away from the engine I used a twisted pair cable for the connector. Even better would be a twisted shielded pair with one end of the shield grounded.

R.