Dear Mr.Beede,
I have looked for the correct boots for the AP Lockheed steering joints, several times over the years and met with no success. I believe there is a reason for this: The AP Lockheed Service Instructions emphasise that where a boot has been damaged then the whole ball joint must be changed regardless of whether or not any play is detected. This might explain why AP Lockheed may never have allowed replacement boots to reach the market. As you say there are perfectly adequate alternative boots available which will fit the TA89 and TA90 steering joints on the Ace/Aceca & Cobra with Bishop Cam and Peg steering box - the only difference being the maker's name. I did put a ruler on an original joint boot and the smaller hole, for the taper, is ½" and the larger, for the groove in the housing, is 1¼" so a little larger than the dimensions you quote, although being flexible rubber I suspect various sizes would do the job of keeping the grease in and the weather out.
I wonder which Cobra you are working on, might you share the car number with us ?
Barrie