Denny, as a backyard engine builder, it sounds more to me like your stovebolt engine is out of balance. Cyclic vibration at around 40 mph. It could also be an out-of-balance driveshaft. Sometimes that is called the "wobble of death". Or it could be tires out of balance.
You can isolate them. If it's the motor, you should be able to hold it at that rpm in park and tell if it's vibrating. Correct me if I'm wrong but it does seem like because of the low rpm range of the stock stovebolts, balancing the rotating assembly might not have been a priority in the 50's.
If it's the driveshaft, you should be able to coast at 40 mph with the tranny in neutral and the vibes should be gone.
Just some thoughts on how to get to the bottom of that.
I can vouch for the rough rider, rattle can quality of the 3600. It does seem like mine rides so rough. Part of that I think might have been the springs that were probably upgraded to handle an increased 3/4 ton load. And my long bed is beat to heck. You can tell this truck has had a hard life.
I might be forced to get parts off of a full-size passenger van with 8 lug wheels. If you think about it, you should even be able to get the 8 lug disk brakes too, as well as the IFS. They were almost always RWD, weren't they?
Anyway, I've been thinking about it.
And gee whiz, try to find some GMC parts in these restoration catalogues! You guys with Chevy's have a lot more available!
But it's a labor of love, right?
