First off welcome to the site from California. On the Holley web site under
Carburetor Instruction Sheet, for part # 12-802-1 (blue pump and regulator for BB motors), you will find the
instruction page that states.
"PUMP MOUNTING AND INSTALLATION:
The pump MUST be located below and as close to the tank as possible. This is necessary to allow for an adequate fuel supply.
The pump is designed to push fuel and not designed to pull fuel out of the tank. The pump should be mounted on the chassis,
in a vertical position with the pump motor on top. Avoid contact of the pump and fuel lines to moving parts and to any hot 2
areas, such as the exhaust. The pump should not be mounted in a closed area, such as the vehicle's trunk. Follow the steps
below for mounting the pump".
Manufactures
always put what is best for the product that they are selling, hence the statement "as close to gas supply and "pump is designed to push not pull". If your friend has it mounted on the firewall and has been running without problems, you can do the same thing. The only thing that would matter is if it went out, and you asked for warranty and you told them you mounted on the firewall they might not give you warranty.