Slant 6 confusion

I have a 79 d100 with the slant 6 Holley 1920 1bbl. I was going home the other night and the truck was running great all that day, then it started spitting and sputtering until it left me stranded. So the next day I took a look and my carb and fuel. Everything was clean and in spec, spark was also good. But when I turn it over it only backfires out the exhaust. I checked valve clearance about 4 months ago and everything was in spec and I’ve been driving the truck about everyday since then. I went back and checked for any wires that could have been touched on the ignition system and everything was good. Good spark on all plugs. So I’m believing that the engine has jumped timing or lost compression. Because I have great fuel and spark but it’s acting like it’s firing at the wrong time. Is there anything else I could check? It’s still sitting on the side of the road at my grandfathers house so simple things I could check first before I have to tow it to my shop. Thanks guys.