Actually a wide ratio makes a big difference in that car, or about any car. Has yours been converted to wide ratio, because big input (1 3/8") trannys never came that way? They're easy to convert though, so maybe yours has been?
Like the others were saying, lots of stuff can cause it. Check all mounts and bushings, even on the rear control arms, making sure nothing is worn or sloppy. Bad rubber bushings can easily cause it. If everything is good and solid, then it's time to look at the flywheel or clutch. Did you surface the flywheel when installing the clutch?