Good luck.
I'd think that the 73 bumper would be your best bet. That's the year they changed the taillight panel. It's also the year with the giant rubber blocks on them. The blocks can be left off without any problems other than making sure you have chrome covers for the bolts.
Check to make sure that the bumper mounts from the 73 will match up with the 75 body. The mount is probably different as the 73 had to satisfy an slower impact speed than the 74+
Anything later than 73 is going to have the 5 mph, buggy bumpers on the rear.
