What they are trying to say is that 1973 model Pintos came with the 1.6L or the 2.0L 4 cylinder motors. The 2.3L 4 cylinder wasn't introduced until the 1974 model (the same 2.3L motor used in Rangers).
That being said, there are no 2.3L parts, aside from the air filter, that will interchange with pre-74 model Pinto motors. Not to sure how your 2.0 headers, that you have on the ranger, are mounted, but I'm sure we would all like to see! Perhaps they were advertised incorrectly? (Just a guess)
Unless your 73 Pinto's 2.0 was swapped out for a 2.3, you won't be able to use the 2.3 head on your 73. They are of completely different configuration.