I'm a pretty decent amateur auto mechanic, but I'm only a beginner on A-4's, and I've learned that there is no resemblance between a car engine and a marine setup. My only credentials are that I own an A4, too, as of the past year. Since nobody else has replied, I'll give you my limited knowledge on this. I had a similar problem. There was excessive vibration at the stuffing box. I knew the vibration was coming from the dive train, somewhere from the transmission aft. In my case the solution was multifactorial. I had a folding prop, which I replaced with the original solid prop. The cutlass bearing was a little worn and I replaced it. The engine was moderately (0.020) out of alignment and I corrected it (to 0.002). Although there is still a fair amount of engine vibration through the hull (my motor mounts are bolts on solid steel angle iron mounted to stringers on the hull, with no rubber anywhere), the drive train vibration seems to be acceptable after correcting the above issues. There is still some stuffing box motion at idle speed, but at speeds much above idle, it smooths out.