I have installed AC Delco's. I have a weird fuel pressure thing that I need to get more information on. Right now every time I open the gas cap it sucks a lot of air. I do not know where to start on that one, I did how ever loosen the cap and ran that for a couple of days. With that I am get a back fire when she is under a load like going uphill or towing my camper. I have the timing advanced about 4 degree via the timing light I do not know how to get the advanced timing while it is running under load. Please explain.You can decarbonize it but you do run the risk of a piece of it getting stuck between the exhaust valve and the seat and then you would get burn thru, resulting in you pulling the head and replacing the valve. But, on the other hand, if you don't, a piece of carbon can dislodge itself, resulting in the previous statement.
It looks as though you tried to retard the timing to alleviate the spark knock. Try swapping to a "cooler" spark plug, AC Delco or Champion. No NGK's or Bosch. Also, check your fuel pressure. If you run lean you will cause spark knock under load too. What temp is the truck runinng at after it's warmed up? What total advanced timing is it running under load? There are a few more tests you need to run to figure this one out.