does anyone have or know where I might buy a new diff. cover for a 67' mustang,6cyl., 8" rearend? can't seem to find one anywhere....

 

There is no diff cover on an 8" axle. The differential assembly installs into the axle housing from the front, almost exactly the same as it does in the 9" axle. Neither the 8" or 9" Ford axle use a rear cover like the 8.8'" axle does.