Mine also would not suck up tight to the rails on my 69 Firebird. I made some shims out of 1/8 inch aluminum to fill the gaps on the bottom side of the frame. In my subsequent teardown for the next chapter, I came across some loose screws. It seems the self tappers don't have much to bite into on a 50 year old frame that was thin to begin with. I have installed nutserts in the frame and use grade 8 bolts to be able to really snug it up now.
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Chris
1969 Firebird: injected Pontiac 462ci, T56
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