This would also mean filling the mustang vibrato routes and re routing. Its just not practical IMO. Unless you're a genius with woodworking and have all the tools and have experience with this kinda thing. Which would also mean cutting a new pickguard too.
The amount of crooked bigsbys, jazzmaster trems and phucked up looking home built guitars I've seen online always makes me think "people don't think before they act" and that they certainly don't research it anyways.
Making guitars (or major mods like this) isn't for everyone, i don't know how to make them but I know enough about making them to know its not as easy as 123 and you cant just wack any neck on any body and expect it to intonate.
You should just get a CIJ/MIJ mustang neck if its a CIJ/MIJ body or a vintage mustang, jaguar etc (as Fran said) neck if its a vintage body as it will be compatible.
Besides,shortscales rule and you'll have a better guitar anyways!!!