Cab vs. frame Registration question.

[Phat Bastard]

I have a 1968 short bed with a cab, bed and rolling chassis and a 1968 complete long bed that was bought for parts that I'm in the process of stripping. My question is which vin #'s do I go by when the short bed truck is complete, the frame or the cab? The long bed cab is in better shape but will the vin # raise any red flags on a shortbed chassis? Thanks in advance for any replys.