I can tell you that my 71 has 1/2 and 5/8. I do believe that they changed in 73 to 5/8" for both. However, either of the heater cores will fit from my experiance.
Like I said, my '74 has a 1/2" and a 5/8". It was like that when I removed the original heater core, it was a 91k original mile, 3 owner car with the last two owners only having it for 2 years or so. But it's a non-A/C car.
And my '71 core has two 5/8" connections. It's an 82k mile car with what appears to be the original core. One of the hoses still has all the factory markings believe it or not. But it's also an A/C car.
I think the difference has more to do with the A/C vs non-A/C cars. Probably by year too somehow, but it's obviously not a clear cut 73+ deal.
