Ah the wonderful Lp powered 4.3... Check your compression with three steps. Dry, wet and than leak down. If this engine was not built with the prestolite (think that is the right name) valve seats than I suspect that you have burnt valves and valve seats. Had a number of failures of valve seats due to higher heat of LP gas ignition. The trucks would be hard start, act like they needed tune-ups, spit burp and backfire.
Good luck and let us know how you make out