If you used the center type compressor, it should drop out through the hole in the lower A-arm. You can't use the external type spring compressors with the Mustang...you won't be able to get them out, if you can even get the springs in.
I usually don't use a spring compressor at all, but rather take out the A-arm pivot bolts and lower the inside of the A-arm with a floor jack, install the spring then jack the A-arm back up into position.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team