I'm preparing to put the big block I built back in 1987 back into service and am have done some research on water pumps. It turns out, there were 2 different pumps in use back then. The 68-69 t-birds and lincolns had a shorter "snout" (drive flange to mounting surface) than the C, N, and J code 429's. What's interesting is the shorter snout was also used on the Boss 429. My engine, a '68 429, was pulled from a 74 cougar, and was a mixture of lincoln and t-bird drive components. I traded a nearly new 3310-2 holley for the car, drove it home.
The other part of this, which is relevant to this post, is that the long snout version was carried forward and is readily available but the short snout is a rare pump and harder to find. Getting the proper date code for a concours restore may also be challenging for the long snout....that I don't know.