1966 Rubber Bushings In Front Suspension

The upper control arms never had 90 degree grease fittings from the factory so those bushings almost always went w/o maintenance. Some mechanichs used to drill access holes in the shock towers that would permit a grease gun to attach to the zerk fitting that came straight out of the bushing. It could be that but those aren't "rubber" bushings at that location. If you have a squeak it could be something as simple as the rubber bushings squeaking under load. One way to solve that is to use teflon tape between the rubbed bushing and the metal. Have you replaced any bushings lately? Is the squeak isolated to either pass or drivers side?

Start trouble shooting by location and go from there.

You need to get the special "Mustang upper control arm shaft" grease fitting kit. These are low-profile extended fittings. No drilling is required, the shaft caps already had plugged holes in them. Many times service stations cut holes in the towers to do this, which is a really bad idea. If your guy thinks there are rubber bushings up there, it means he is used to Camaros, not Mustangs. The all steel Mustang upper arm shaft is more stable than the Camaro arm, but left ungreased, squeaks are practically standard equipment.

Check your spring perches too.