In 1980 I bought a new frame from GM for my then 1969 Corvette (the 69 frame was slightly bent). In 2008 (?) I decided to use the frame. The 69 had long since been stolen and I had bought a 1970 BB Coupe as a replacement. 28 years after buying the frame, I decided to populate it with the forward subframe pieces, etc for my 70. Things were not fitting. The various Corvette Vendors were not helpful, although they tried a little. Finally, I called Volunteer Vettes...they sell among all the other stuff a lot of frame pieces. They're in Tennessee. Amazing, they took my personal fitment problem as a challenge. After a few back and forth telephone conversations, what they discovered was amazing. In 1980, if you would order a C3 frame from GM, you would be sent a 73 frame! The 73 frame was not compatible exactly with the 68 (of course), and it was also not compatible with the 69 through 72 cars. The 73 had a plastic front bumper. The 73 frame came close to being compatible with the 68-72 cars. In 74 the frame was changed to be more compatible with the plastic front bumper, the 74 frame is not easily backward compatible with a 68 to 73 C3.
If you have a 68 through 72 Corvette and end up with a 73 frame, a body shop can pretty easily make the changes in bolt hole spacings to get things working for you. My congratulations to the person at Volunteer Vettes who worked this all out for me!!! I bought some frame parts from them, but I don't think their profit paid for all the time they spent working on my problem.