I think the problem may be that no one here has done it, which is kind of amazing, thus they don’t have an answer. The Chevys changed the frame style in 1963 to the more modern rails without the X. Your ‘66 cab came from such a frame. If the GMC frame is the modern no X frame I would think it would swap. Might take a bit of bracket modification and some welding skills but anything is possible. I say go for it.🛠

Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne (Emily)
‘39 Dodge Business Coupe (Clarence)
“I fought the law and the law won" now I are a retired one!
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USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)