You got the spark wires on the dist in the correct order?
I am unfamiliar with using propane. However, to keep thing simple, let's take the fuel system out of the equation by just pouring a bit of fuel down the air cleaner pipe and turning on the choke.
It run? Yes - then

No - verify timing
Bad timing - fix it.
Good timing - check compression on all cylinders. If no compression - check valves (hanging/sticking open) and then head gasket. (I had a lousy sputtering 230 - cyl #6 piston was discovered in the oil pan

Your good vacuum indicates the engine is in good condition and valves close/open well.)
Funny story (now) - Spark plugs - verify they work. I got 6 new NAPA plugs and 4 turned out to be bad. Gave me fits figuring out the problem - until the wife suggested putting the old plugs back it - of course, it promptly started and ran great. I have no idea why the 4 new plugs were bad.
Got decent spark at each plug? No - find out why
Yes? Then the problem is either bad fuel or camshaft flat b/c you ruled out the other big possibilities.
Hang in there. Your engine is very simple and forgiving. It wants to run.