I have seen these measurements in the MPC too. BUT, think about this! The GT (slotted as some call it) wheel is held down by only a little plate and a wing nut, yet they list it as being longer than the hold down for the regular steel wheel, and the regular wheel is mounted with the jack handle/lug wrench on top of the wheel, which means to me that a longer bolt would be needed, not the shorter bolt as is listed in the 1975 MPC that gives the dimensions listed in the post above. This makes no sense to me! I have posted here in the past trying to get someone with the factory issued GT wheels to measure their hooked bolt, photograph and post an image of the hold down plate, and clue us in on how the factory equipped those cars with the slotted wheels.
You are correct, no one that I have been able to find sells the correct hold down bolt or plate for the GT wheels. HELP! This is driving me nuts. Who has an original verified 68 with this option and can or will bring this question to a resolution. PLEASE, a nice image with measurements of both and hold down bolt and the plate.