I have more than a dozen 'collector cars, including 6 TriFives. I purchased all of them on their original frames and with original VIN plates. The 1957 I'm currently working on now sits on an aftermarket chassis/frame. In doing that, when I removed the factory frame, I photographed the original frame, with closeups of the VIN on the frame. I have the receipt where I purchased the aftermarket chassis. I believe with this information, my 'legal ownership' of the car could/would NEVER be questioned.
PS. I SOLD the original frame/chassis from the '57 Nomad, but when I did I 'XXXX'ed out the factory VIN on that frame to prohibit anyone in the future from attempting to register another car with the VIN from my car.