if you are getting gas and spark it should run, nbut somewhere in there the system is breaking down. usually it is a systematic process that things have to be checked. putting in gas treatment will just take your money. compression check will tell an ideal of how the top end on your engine is. burnt valves will cause backfiring as will ign timing. you say that someone installed a HEI system in your engine. a lot of people over look one item when they installa het in these vehicles and that is that they do not remove the resistor wire or resistor coming to the ignition that was used with the original system which supplied 9 volts(resistor knocks it down from 12). HEI requires a full 12 volts. you eliminate that resistor or resistor wire with a new 12 guage wire.
.finding these kind of problems are just a process of elimination. if you are going to keep the truck, buy s few tools that will let you check things for yourself. if you have any interest you will learn. most of these testing tools do not cost that much, but thety will simplify the process. timing light , compression guage and a pressure guage foe checking fuel pressure from the pump.
Bart gets a little exuberant sometimes
.your diagnose doesn't really give anyone really the info to give you an answer online other than a guess
sometimes you just have to get a little grase on you
ron
