With clocks in 289 Cobras, there are two paths depending on whether the rest of the instruments are Smiths or SW.
If Smiths, then you need to be looking for a 60mm Smiths clock, a CE.2194/0X, which was used on a number of Rootes Group cars. There is another 60mm Smiths clock which looks better (less ‘messy’ than the CE.2194/0X) but not sure where it came from, and then there’s another alternative as well which is a 60mm Kienzle clock, looks very much like a Smiths design, and I have a picture of one fitted to an original car. I have one of these; it came from a 60s Rover 2000TC so is period correct. It needs a chromed double-vee bezel to replace the black single-vee, but these are available from CAI (the guys who make the Smiths gauges).
The last option for a Smiths instrumented car might be to use a 52mm Smiths clock, available from Nisonger, but to my knowledge this size Smiths clock wasn’t used on Cobras.
See:
Nisonger Instruments :: the exclusive Smiths U.S. Warranty & Service Center since 1949 (its down at the bottom of the page)
If the other gauges are SW, then it seems that the Galaxie or Falcon round clock is the one. They do look a little cheapo though - my apologies - but that’s what the later SW instrumented cars had.
Cheers,
Glen