Running extremely lean!

[brothers69c-10]

It's been a while since I've been on here, but I'm back with more questions. Some of you may remember my old 78 K10 that i had a 427td in. Well i finally got tired of trying to get it right and found a good deal on a 454 with high miles on it. Never heard it run, but the price was right. I put it in 3 weeks ago and got the truck running and driving. The motor ran better then i actually expected it too. This last weekend after i put some miles around town on it, I took it on its first trip over 15 miles in 3 years. Truck ran great, drove great. Half way to my destination it started running rough, like it was struggling. It acted like it was overheating/vapor locking, or even a possible vacuum leak. It never died and it idled fine. Now this happened when i was doing about 55mph down the highway at about 2000rpm's.

So i checked the obvious, made sure no vacuum lines came off or cracked and they were all fine. Next step was change spark plugs. The plugs that were in it when i bought supposively only had about 2000 miles on them so i reused, but they were black as coal when i put them back in. When i pulled them out after it started acting up they were all white like a brand new one. I put new ones in anyway and that had no effect. The fact that they were white and the engine was running very lean really surprised me since i have a 780cfm holley on 2oo and some horsepower motor.

Tonight I changed the carberator out with an old quadra jet and that really didn;t have any effect. I don't know if the quadrajet is any good or not, but since there was really no change in how it ran, I'm guessing its not the carberator.

Anybody have any ideas? Its really hard to hear any vacuum leaks thanks to the crappy headers I have on it and all the exhaust leaks. I'm going to get a can of brake cleaner and spray around while its running to see if i can find a vacuum leak or not.

Also forgot to mention that at idle in park the motor runs just like it did, firing on all 8 cylinders. Once in drive it starts surging a little. Then when trying to take off or diving down the road it spits and sputters and every now and then it will kick in for about a second and run normal.

I think this truck is just cursed, but any and all help/advice is appreciated.