How to tell what year frame I have

[RustMaster87]

I purchased a frame and body from two different people so I have no idea as to what year my frame is. All of my body panels are 73-80. My frame has the cross members that have the round cutouts for reduced weight. Other than the cutouts, how can I determine what year my frame is? I am trying to avoid wasting money on the incorrect body mounts and checking to see if I have to modify anything else. I searched but came up with lots of information that was hard to sort through. Thanks for the help.