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 Coupe
Black w/no Mach 460, 5-speed
No. 1118, VIN 1FAFP42X21F213026
Delivered 9/11/2008 w/47017 miles