88 caprice vibrates roughly at stops & in park only

i have a 1988 caprice classic 305 5.0 v8. my car seems to run really good while accelerating but when i come a complete stop or in park, it vibrates roughly, now i recently replaced the plugs, plug wires, cap, rotor, external coil and redid the ignition timing and have the plug wires tested for resistance and replaced pcv valve (which is the correct one and works properly) and pcv hose,spark plugs gapped properly, all igntion is very good, im assuming its probably a vaccum leak of some sort but i cannot pinpoint it if it is. i sprayed carburetor fluid on the carb and all over the engine hoses etc.. to see if the idle would change and still vibrates roughly. i dont hear any whistling or hissing sounds of a leak at all. the carburetor has been rebuilt and tested and that didn't do it. one thing i did notice was i had a wheel out of balance on the front driver side and it did make my transmission gasket come loose and i had to put a new gasket kit and retightened the bolts, is it possible that the unbalanced tire also could have caused my motor mounts to go bad? And what causes the engine vibrate roughly only in park and or at a complete stop? i really need help with this.. i tried so many things and haven't got any answers to why its doing what its doing.

the engine vacuum is checked and leak locating by a vacuum guage....you place the gauge with the proper fittings in line with the line being tested then sqeeze the rubber lines to see if the vac changes...this is the only way ..using your ears is weird...using carb cleaner on these rubber hoses and other parts is destructive...then with this age the carb base gasket..
the egr may also be slightly stuck open...

those carb vacuum operated devices are known for leaks...

the engine mounts will cause an engine to move too much ,,,and the transmission mount as well....another problem I ran into was the motor mount frame brackets were worn where the bolt ran threw,,had to use a larger diameter bolt,,and file the holes slightly,, for a snug fit..

driving with tires out of balance will damage the front end ...wheel bearings ....etc..

Is it possible someone has installed the wrong flexplate or balancer? Or maybe the balance weight has come off the flexplate. I had a 77 Grand Prix with a 301 that had the same vibration problem and finally figured out that someone had replaced the flexplate with one from a 350 that was for an internally balanced engine and the 301 is externally balanced.

Is it a miss in one cylinder?