Screw in ball joints were originally Chrysler products; Moog #719 or 772 (usually upper) and 727 (usually lower). For Cobras I would avoid the replacement market (except for MOOG or TRW) and go with Coleman Racing. Much higher quality, and a lot more travel before binding. Also helps with alignments. Check the sizes with a micrometer first--I have seen 719/772's used on both upper and lower. If you use a Mustang II spindle, it would have the K772 to use the correct taper. Readily available at any good parts house or Coleman Racing.
Ball Joint, Mono-Ball, Upper, Chrysler, K-719
Good luck!
Robert