Ramon - thanks to you and to your restorer for this information. However, the source of this LED set (XKs Unltd, but Welsh and others advertise the same thing) says that these LEDs are "polarized" (if that's the correct term) for positive ground systems. They also advertise taillight LEDs for both positive and negative ground systems.
Have him check these supplier's websites and/or talk to them directly to confirm. XKs assured me that these particular bulbs were "plug and play" for my positive ground system (if only I could get to them!).
Thanks again.
(p.s. - you may know this car - it was my Dad's and he's in the F'ville/Manlius area. Who's working on your car?)