"Looks good" is not a suitable test of motor mount condition. Since you've already stated you've got a SM-319 overdrive transmission, ignore all the advice about the 4 speed having a low granny gear- - - -it doesn't apply to your situation. Try this as a quick diagnosis procedure- - - -NOT a permanent fix- - - - -attach a cable come-along between the driver's side frame rail and the exhaust manifold, and put a little tension on it- - - -not a great deal, just enough to keep the engine from rocking back and forth as you accelerate. If that eliminates or seriously reduces the shudder, change ALL the motor mounts, front and rear. There's no need to add a rear crossmember that the factory didn't install in the first place. Good luck!
Jerry
