Here's a picture of the actual stud off my 66. 1 9/16" long, 3/8-16 block side, 7/16-14 connection side.
Dave
This is one of those poorly documented (from a restoration standpoint) areas. It involved three different Ford engineering groups, engine, automatic transmission and electrical, and they didn't communicate well.
The stud is not shown in any of the Mustang Assembly Manuals but is shown in the Ford Car Parts Illustration Section A71, page 6, as part number 380818-S, and it is indicated as "not serviced". The part number is not in the AMK Products Guide to Ford Fasteners, nor in either Ford Standard and Utility Parts Catalogs, and no photo or drawing for this type of stud is shown. The dimensions I have are 7/16-14X1/2, 3/8-16X3/4, 1 5/8 oa length.
From what I've determined, the stud may or may not have been used on the 170CID I-6, all that is shown is an existing stud (see E4-8125-1, 64 Electrical Manual) for the battery ground, with or without an AT.
For 65 thru 68 200CID I-6 engines with automatic transmissions, it was used per Illustration Section A71, page 6.
For 66 thru 68 289CID V-8 engines with automatic transmissions, it was used per Illustration Section A71, page 6. It was not used in 1965 on 289CID V-8 engines with AT's, at least on my May build San Jose or on a January San Jose build.
This stud is different from 380818-S in that it is threaded 7/16-14 on each end which is what I've found on my two 66 Mustangs and a couple of others with 289CID V-8 engines. The V-8 battery cable has a 7/16 dia lug.
I have a couple of the 7/16-14 - 3/8-16 I-6 studs (the one shown, thanks Dave) available already clear zinc plated with the appropriate two washers and nut. Send me a p/m if you are interested.
Jim