Thanks for responding.

Some information did not appear in my original post.
Let me clarify:

Make: Yale
Model: GLC120MJNGAE084 (propane)
Serial: D818V02027C
Hours: 8381
Engine: Vortec V6 with Impco Regulator/Vaporizer; Impco Lockoff not used.

I will answer your questions now:
Did not originally adjust air/fuel mixture at carburetor; since I found that Plugs and Wires were old and worn, replaced them first.
(Was able to replace Distibutor Cap and Rotor today, also replaced LPG Fuel Filter).
New Plugs are gapped correctly.
Did not find any vacuum leaks.
According to the forklift operator, backfiring seemed as though it would get worse as the engine heated up.
Was able to remove cover of carb/mixer without the screws stripping out the threads. The diaphragm looks fine. Reinstalled diaphragm/ cover.
Spark quality is very good, with new parts.

Today's results:
Engine started right away (with the new tune up parts) and remained running approximately 45 seconds without backfiring, then it shutdown.
Attempted to restart engine WITHOUT ether/ starting fluid at 3 separate times. Engine rotates, will not start. Plenty of propane in the tank.

This new problem reminds me of a bad Regulator/Vaporizer that prevented Propane from reaching mixer.