1967 Driveshaft

First of all is the front u joint the same as the rear u joint? It should be ,unless you have a 64 1/2 4 speed .The 65-70 Small block 4 speed is a 28 spline output shaft ,but the 64 1/2 has a 26 ,it think it is rear shaft spline size.
67 drive shafts were the first to use a press together drive shaft with a rubber coupler for vibration but even that had same size joints front to rear . A step down shaft is not correct for 67 but will work if the correct length and u joint are the same front to back. Next question is what rear end is in the car 8 inch or 9 inch they may have the wrong yoke and had to make a drive shaft . Do you have a measurement for your drive shaft ? I have a 67 shaft i will measure and get back to you .
There is another possibility ,it could be a Torino 4 speed ,they are longer than a mustang 4 speed . Can you get a length measurement from the front mounting flange to the end of the tail housing of your 4 speed ?

Well the pics didn't turn out real great but ,this is a 67 early drive shaft .It is the one piece shaft and has the same size joints on both ends .it measures 50 3/4 inside of cap opening to inside of opposite cap opening , it does not have a step down .

The measurements on the 4 speed is 24 inches from the front of the mounting flange to the end of the tail housing ,not the shaft .