What a pain. Yeah, a 1/4 to a 1/2 inch here or there is not uncommon, but the gaps you're dealing with are pretty severe. The factory shims are pretty thin, but typically stacked several deep to get the desired size. Maybe up to about 1/2" or so from what I've seen. Maybe more if it's really needed.
For some reason, I seem to remember seeing most of them on the passenger side front mounts most of the time. Both my Bronco and my '79 pickup had them under the right core support, and the right front cab mount (under your feet) but I don't know if that's always, or just the few times I've found them.
It's very possible that after all these years your frame is tweaked. I don't know how to check it, but I remember someone posting (a long time ago now) a spec sheet with all the body and frame measurements. Maybe a deep search will turn up something. Or hopefully someone will have that info handy.
It's also conceivable that your body is slightly off too. I imagine they take pretty good care to get them straight, but if it's a bit off, and your frame is a bit off in the other direction, you'd get this kind of problem. Is it possible that your front steel and main fiberglass are not quite matched up? Can you set up some little stands on flat ground to verify that the body's mounts are flat? Can you measure up from the ground to the tops of the frame mount pads to verify that they're all the same distance from the ground?
I don't know if that's accurate, but it seems like it would help get you close.
If you can't measure them to verify factory specs, you might just have to build spacers yourself to compensate for the gaps. Getting the body panels straight seems to be the prime goal here I would think, so a spacer to accomplish that would be important.
Good luck.
Paul
