You can check the thread in post #4, it explains in detail what the problem is with the support for 68 and 69, I'm not sure if it applies to later models.
The OEM support in 68 and 69 is at a 10 degree angle forward at the top. The bottom rail should made so it is flat with the ground when the support is tilted 10 degrees, instead many of the repops have a bottom rail that is parallel with the top instead of at an angle. They can be forced to work, but it screws up the core support to frame bushings and makes things like fender alignment a real pain.
The 70-72 have substantially different body work than the 68 and 69 ... could be the repops are made for those three years and vendors just assumed they fit the earlier years too. Have the current repops been retooled? I don't know, and for some reason all the vendors seem reluctant to address the question.
I don't have a 70 or later model to compare the OEM pieces. One thing you can check is ... look at your OEM support ... is the top slanted down in the front? Is the bottom rail parallel with the ground front to back? (can be hard to tell since it's narrow). If so, then you could have problems with the repops.
This thread
http://forums.maxperformanceinc.com/...d.php?t=835789
has quite a few pictures which should clarify the situation. If you are desperate for a support ... sounds like you probably have the necessary tools and skills to modify the repop to work anyway. That's what I did.