You'll have to make sure your bridge is positioned at the correct scale length, assuming the neck is for a 34" scale bass and the unknown mustang body is actually a 30" scale.
Other than that, I'm sure you'll come up with an instrument you enjoy. It can be almost guaranteed. There is a certain magic that surrounds an instrument you have assembled/built yourself. Put it together, give it a setup, then play it. I'm sure you will enjoy your project.
The "Official" Black 'n' Maple Owners Club - Member #216
The Kid Bassists Club - member #4