I had a chance last week to check the sending unit on my January 31, 1967, Dearborn built C Code last week. It's the same size as the one on my '66 (3/8" PT, takes 11/16" socket), is also stamped "260", but also "7A' after that (which coincides with my build date). However, instead of the script "F", it is stamped "AUTOLITE". I of course forgot to measure its resistance - next trip.
I also had a 4bbl intake that I checked out. It is earlier (intake is cast "C40E-B) as it has a sending unit with 1/8" PT, that takes a 1/2" socket. The sending unit appears to be an aftermarket (no "F", FoMoCo or "Autolite" on it), so I didn't investigate it any further.
Bob