A mustang with the original style bridge with large plate will need 32" medium scale strings as it is string-thru, but the ones with a standard BBoT will need 30" short scale strings.
I have a few shortscales with various different strings. My favourites for the mustang were TI JF-324 but then I added a JMJ mustang to my collection which came with standard fender 9050L long scale flats which sound and feel great so I kept them on the bass and got a set for y other mustang.
I recently bought a Guild starfire bass which also needs medium scale strings despite haveing 30" scale length. As it comes with roundwounds for some unknown reason I needed to source get some flats for it and ended up with status stainless flats. These are made by picato I believe, and no longer being made so stock will run out at some point which is a shame as they are perfect despite being half the cost of similar strings.