Not so good vibrations...

Right, I got this G20 running and all tested and set up. It has a new SBC specially built for good torque to shift this heavy vehicle, new trans (TH700-R4), freshly balanced driveshaft, new torque converter. It runs fine, but vibrates too much for my liking. It vibrates so hard at some speeds that my CD-drive skips. This seems to be an engine thing. When travelling at high speeds (over 60 mph :D ) it shudders, but if I coast at the same speed it doesn's so I guess it's not the wheels... All other turning things are new. Could it be the cooling fan? Then it would surely do it sitting in the back yard, too, wouldn't it?
Next probelm is, if I really step on it, it rattles. Sounds like something is lose, but actually it can't be, control arms have new bushings, all shocks brand new. This isn't consistent with engine speed, but comes with acceleration near WOT :CRY:
WTH is this?
Maybe some people pay money to have massaging seats in their ride, but I just want it smooth :3dSMILE:
Any help is really appreciated,
Mad

It's probably not the case, but I have to ask anyways, are your lug nuts tight? No offense intended, but sometimes it can be the stupid stuff. I've done it myself.

The rattle when you really get on it could be from the exhaust. Once, when I replaced my solid motor mounts with some rubber ones I got some weird noises upon really hard acceleration. It turned out to be a pipe hitting the trans crossmember. The clearance was really tight and the torque moved that pipe just enough.

I've had both things myself, once some lugs on a rear wheel not tightened properly and another time my newly built exhaust didn't clear the trans crossmember by enough. But that is ruled out already. I guess I might get my wheels rebalanced, the driveshaft is straight, U-joints are new.
But another question about this rattling: How would timing create a rattle? How would I change that situation?
Thanks for your help
Max