A mustang neck will not fit correctly on a Fender Stratocaster. Mustangs use a 24″ scale neck and strats use a 25-1/2″ scale neck.
If you want a shorter scale neck for a stratocaster check out the Warmoth Conversion neck.
[From Warmoth Site]”By shortening the neck and compensating the nut location, this conversion is possible without any modifications to your present bridge, pickup or neck pocket!”
Warmoth 24-3/4 Scale Conversion Neck