POS GM G-Body!

Its been awhile, but i figured i'd vent on the mopar board, Heck its a GM bashing thing lol, I decided to build a GM this time around, found a fairly clean 78 Cutlass 2 years ago, Installed a 406 sbc, well wasn't fast enough, sooooo i went & built a 469 BBC, Installed a 9" with a spool & 3.73 gears, PTC 3000 stall, Car runs great, They problem is it has shaking in it at speed i can't for the life of me get rid of, i have done it ALL, from front end all the way back, going over & over till i'm blue in the face, I'm sick of it, Has anyone ever had shacky GM cars in there time, maybe the Mopars guys might have some ideas.