Hey...I'm the guy from for a bodies only forum. I have a '68 Barracuda 383, 4 speed, 3:23 sure grip convertible. Unlike Tim's car, my car is not numbers matching. It is an original 383, 4 speed convertible car but it was missing the big block and 4 speed and the big rear end. Anyhow, everything on my has been rebuilt or is new. Had the tranny rebuilt, got all the rest of my 4 speed stuff from Brewers. Had the 3:23 suregrip rebuilt, etc. My car acts exactly like Tim's. I originally thought the clutch was the problem and replaced it (new fly wheel). I recently filmed the rear in slow motion with my phone in my driveway and the leaf springs jump up and down pretty violently which is what the shaking is. I was about to buy new springs when I read Tim's other post and saw that he tried that and it didn't help so I don't know what to do. I put this car together myself and it's super frustrating and embarrassing that I have to take off with no acceleration until the car is rolling or it will shake pretty badly. Someone else must have had this problem before.