I am not feeling like this an ignition question, but like Asylum getting the impression that its more of a carburation issue. Has carb been gone through and set?
I really don't think with that set up the plug gap difference would cause the truck to get bad economy or diesel.
Dieselling can be caused by a number of factors - if the truck runs really hot - or is really carboned up - then that crud can carry heat and make the thing continue to run. Trucks will run hot with too much advance, but you don't have that problem at 4 btdc. If the carb is dumping fuel, that might be an issue.
One thing you can try is to see if the truck trys to stall if you start quick or stop quick - which would indicate sloshing of fuel in float chamber - i.e. floats not floating or set wrong. I just went through that.
Also check your plugs after use. They should be light brown. If your plugs come out black and wet all the time and its not appreciably burning oil - that would indicate too rich of a mixture. Too rich can also be caused by a dirty air filter acting like a choke, thus richining up the mixture. Engine starved for air - bad economy.
Your carb adjustment on idle screws should just be to adjust to highest idle, set about 1/4 rich from there, then re-adjust idle speed. If it won't idle, like Mr. Asylum sez, then there could be a vaccum leak causing the mixture to get goofy. Then you'd be leaning on the gas to make it go, thus, bad economy.
I really don't think .030 overbore would change things very much with stock fuel system and stock exhaust. HEI should just give you a hotter spark, which should help burn more fuel, but, not make a huge huge difference as these things were ok spark-wise stock.
Anyhoo - anyone else got better ideas? That's all I can think of off hand.
