Welcome to the forum. I assume you have a HP with a C4. Any San Jose V8 with a radiator for an automatic transmission is correct for your car. The way to identify the radiator is the tank is stamped "W-MO" with the date, month and year in numbers (mine is 9-65), stamped from the inside and read up side down (read correctly from the engine compartment). There "may" be a part number tag on the top, "C4ZE H2".
Jim
I wasn't sure if this specific marking information was in the Forum's Library, it is, under thread title
"ConcourseMustang.com Official 66 Mustang Article List" and scroll down to the date code topic. It is an Adobe .pdf file. You want page 35.