Thanks.

Fixing the fuel issue is about the same deal as with gasoline engines;
Upgrade the fuel lines and in this case also the lock-off valves (there's one in the tank and one in the engine bay.

Although I'm not really quite sure yet the fuelline is the actual limiter.
As propane evaporates from liquid to vapour, it expands some 270 times in volume.

The safety lock-off valves however have a smaller diameter inside and are more likely to limit flow to the propane vaporizer.

The carb used is an IMPCO 425, rated to flow 460cfm, which should be more than enough for this engine and cam-setup for now.