For some reason or other those herc 2.7 disturtors give me trouble. I'd start with a complete tune-up and check your air gap in the dist. (can't recall gap spec)
Your carb should be a impco 100, check to be sure you mix is set right. Also check your regulator. If it's stuck open you'll be flooding raw propane into the engine. Take the hose from the convertor to the carb off, at the carb and turn the ign on. If you hear hissing, or propane sperts out, your converter is bad. It is designed so that propane is sucked out under vaccum, not forced out under pressure.

firing order 1324 or 1342 can't remember
good luck