help needed with 44 WC51

[elvis3006]

hi everyone i have a 44 WC51 that has engine problems.it mostly concerns misfiring at speed.going up hill in 3rd or 4th gears,sometimes even in 2nd on flat ground.i have tried to fix it myself,replaced the coil, drained the fuel filter,points,condensor,dizzy cap and spark plugs.when all this failed,i took the truck to a mechanic friend,as i do not have the various tools or space to tear an engine apart.he has worked on it,overhauled the carberator,and thought he had it fixed,engine has more power but still misfires,backfires under load.he is stumped,i mentioned to him that maybe someone here could help with info.anyone out there with 6 cyl flathead experience? if so pm me and perhaps someone could call him. truck has been at his shop for 7 weeks now,hope to have her running before the muster in aug. thanks for any help. Rob