Hi Le Fou,
The OS 61VF should work assuming the tank is not too far (I don't know how far you can go here) away from the engine. The Tank C/L is above the engine if starting upright, but below it if starting inverted. I know it is a bit of a pain, but I eliminated the problems of starting inverted engines by building a cradle to hold the airplane inverted for starting (regardless of the fuel delivery system of the engine). How far is the tank from the firewall?
The information above is correct, the Atlanta was designed for a the OS pumper or YS (the short stroke version as the LS was not yet available) or Enya pumper.
Great flying airplane when you get the power issues resolved.
-Will