I have seen round tanks rotate from vibration, twisting the fuel and vent lines to the point where the fuel line eventually pulls off of the carb needle. Any mounting technique that would prevent this rotation would transmit vibration to the fuel tank which could cause fuel foaming.

Do yourself a favor, put the firewall back where it belongs and get an engine without the rear mounted needle valve. Or, figure out how to install a square tank in the same space. It can be scotched into place using soft foam sheets which will not let the tank rotate or pick up engine vibration.