I think you'll find the springs are all the same for the C3s right up to the 82 model. The only difference appears to be in the mounting hardware for some models. I have a few copies of VBP's latest catalogues if anyone wants one too.
I have a 330 # composite rear spring on my 77 that I'd like to swap for the 360# one plus I still have the old 9 leaf steel spring laying around here too.