I recommend Cobra Bob's shifter gaskets! I had one on my Mach 1's shifter after it got changed out to the MRT (nee Steeda Tri-Ax) shifter.
I DON'T recommend PY's large shifter ball soft know to damp out noise. That solution is more useful for "other purposes..."
2001 Ford Mustang Bullitt - black, #1118, VIN 1FAFP42X21F213026
2009 Ford Mustang Bullitt - highland green, #6323, VIN 1ZVHT82H995129122