Yes, I used to be the guy with the "Spaceship" C10! LOL! You can see the pictures here
http://www.geocities.com/licken2/Vehicles/index.html ! That truck ended up being sold for next to nothing by my father. It was sitting in his drive, and he got tired of looking at.... so he sold it to a guy he worked with for around $200, who is still driving it (2 years later) !
As far as this mystery sound the 72 has, I think it's been pinpointed. It's definitely not coming from around the dash. I had my G/F listen as I slowly drove around the street we live on. She walked beside the truck. (THat's how slow I was driving). It's definitely coming from the space between the cab and the bed. Seems like a bad carrier bearing... I think I already dislike the 2-piece driveshaft. I crawled under the truck and tried to wiggle the driveshaft up & down, and side-to-side, to see if the U-Joint was bad at all. Absolutely no movement from it whatsoever (Still good?). I'm going to drop the shafts tomorrow, and will most likely do the U-joints too (why not?) while I have it out. Then I'll let everyone know if that corrected it. Otherwise I will most certainly look into the other suggestions with more ferocity!
Thanks for the great advice everyone!