If that Mustang is a V-8, it's a fine donor car. You can use the engine and trans, alternator, etc. If you are converting the Mercury to disc brakes, you can use the Mustang's master cylinder and metering valve, the brake pedal, and possibly the brake pushrod. If the Mustang is fuel injected, be sure to keep the computer and wiring harness unless you intend to change the intake and run a carburetor.

If the Mustang is a 4-cyl, sell it and get a different donor car. The 2300 engine is a good piece, but it's a little anemic in the torque department to pull that big ol' Merc around...