63-66 sending units vs 67 and up

All senders produce the same resistance range of 2 -> 10 ohms.
The difference in fuel senders is due to the difference in tank shape. Somone may have discovered the later sender will work in the earlier tank, like trying on a pair of jeans.
Anyway, the wrong sender will either set pick up screen above the bottom of the tank, show empty at gauge, pick up air , though there is 2 or gallons still in there. which is fine. The wrong sender will bottom in the tank and not mount/seal properly.
And all these systems have a gauge voltage regulator locatedsomewhere.