GM made changes over the years, but there are really only a few sizes of radiator (2,3,4 rows; 17" vs. 19" core height, etc.). The simplest is to match the size of the radiator you have (measure the core, not the entire radiator). My '75 had factory HD cooling, which meant a 3 row radiator with the 350 in that year. They may have also offered a 4 row, but it was probably with the 454. My point is that it is a little different for each year. For best cooling, just look for the best/biggest radiator offered in your year.
The radiator is "clamped" vertically between the radiator support on the bottom and a long bracket on the top. Inside the brackets there are small molded rubber pieces at each corner, and they are different sizes based the number of cores (thickness). If you swap from a two core to a three core, you need to use the right rubber piece that will fit exactly over the corners. Of course you also need to have the same core height and width if you want to use original mounting.
Bruce