heres an idea for you: back in the day when we were converting 116's to 121's, it was all about moving the skid back. if i remember correctly, the rear and center on the pII's were where they needed to be(no center on the 116).
anyways, moving the front point back around 2.5" should allow you to bolt that skid in and run the 121 track, if i read that right.
no cost involved short of a few bolts, mostly labor, if you have hand tools at hand. i have a 121 and 116 tunnel, if your up for it, we can do some comparisons on the mount points and i would be glad to walk you through the details.