Got a good one for you??!!!!

[nicolas72gmc]

Ok fellas, sorry for the book, here is the deal, I drove the GMC all the way to Superior AZ friday. The truck did awesome until about the AZ state line. It started chugging at low cruise (about 35 mph) and at 3/4 all the way to WOT. At 45 to 55 mph it does ok. At 60 to WOT it was cutting out almost to the point of dying, if I let off the pedal a little it would sound like it went back to normal. While it is cutting out if I let off the pedal completely it will back-fire and sounds like it is burning unburned gas in the headers. Sometimes it pops so loud it sounds like the headers are blowing off. I did check the header pipes seperately and #6 is running really hot (600 degrees) compared to 450 or so on the rest. Fuel delivery seems to be fine. I still have the clear fuel filter and can see fuel while running, I even pulled the top of the carb to check for blockage and bowl/float levels. Engine temp stays in the normal range as does oil pressure. I switched over to the HEI this summer and it has ran fine until now. Here is what I have done so far in the last two days:

Changed to MSD cap
Changed to MSD rotor
replaced stock coil
replaced module in Dist.
replaced all spark plugs
added a can of Seafoam to the gas.
Replaced air cleaner
Replaced MSD wires
Purchased the pick-up coil, but don't know how to replace it.

What is weird is that if I put it in park it revs just fine, but put it in drive and take off it does the above again. I am losing my mind here and would like to hear some other suggestions. Thanks for any help Ya'll can provide.