timing, backfire help

[68black_betty]

i have a 454 bored over 30 with flat tappet cam in my pickup, i just put on a holley 750 dp and an edelbrock air gap intake. i got the holley tuned to how i want it using a vaccuum guage, but everytime i get on the highway and get over say 3000 rpm or so, when i let off the gas im gettin a backfire, so i went and put on an advance curve timing kit and an msd super coil and rotor on the distributor, and i am still getting the backfire. ive had the timing down to 10 and as high as 18 if i put the timing up round 20 i get a fireball coming out of the carb, changed the spark plugs because looked like they were foul, was runnin pretty rich might still be runnin rich but im gonna give it a week or so to check the plugs and see if its still runnin rich. the truck runs great gets up and goes i just cant get rid of that backfire, if im only driving through town, and keeping it below 2500 rpm there is no problem. can anybody please help me out?