1995 mustang 5.0

will a ECU tuner allow me to retard the ignition timing?

by tuner do you mean aftermarket? and if so then yes it will. you will have to get the car tuned again though.

why?

The ECU listens to the knock sensor and adjusts the timing to prevent detonation. If you're having excessive detonation, something is wrong, and needs to be fixed before you go off changing things.

You should try a higher octane gasoline or some octane booster.Also make sure the base timing is set a 5 degrees tdc. You will need to unplug the spout connector and get a timing light

Base timing on the 5.0L H.O. is 10 degrees (if you're talking stock). There is no knock sensor on a 5.0L Mustang.