I bought a cast iron pump from Summit yesterday. it seems to be of good quality, but not near the quality of the one I removed.
The pump I removed is stamped Made in USA and has thicker webbing to support the snout.
Look at the top of your WP where it meets the timing cover. You will see the ribs on the timing cover, 2 or 7.
Your 66 289 will have 7 ribs if it is original.
I recommend the cast pump if you have 7 ribs on the timing cover, they seem to hold up better, at least the parts store offerings.
1965 Mustang Coupe 289/C4
1967 Mustang Coupe 200/C4
2017 Mustang EB Premium