[function_seven]

The DMV solves this by saying, "The Frame".

Now I don't know what happens if I cut out the left frame rail and replace it, then cut out the right rail and replace that, then the front sub-frame, then the rear. Whichever section has a VIN plate riveted to it??

[InclinedPlane]

Similarly, the ATF solves this by saying "the lower receiver".

[jheriko]

i would guess that you are actually not allowed to do such work, that is kind of the point... at least here (UK), to be legal i'm pretty sure you need to get a DVSA (used to be VOSA) inspection and approval rather than just a regular MOT if you do work like that.

although the regulations in the US are famously lax compared to the rest of the developed world... so maybe there is just nothing there. XD