If you could find a scruffy one with a bit of rust in the guards in someones back yard that would be a good starting point. You would have to strip it to a bare shell and put it on a rotisery to do it right. I reckon a lot of later model components could be easily adapted to improve the ride and handling etc.
You can now buy reproductions of the Borrani wire wheels that the originals were fitted with.
Dayton Wire Wheels
If I built one it would have to be fitted with a set of these. Aloy wheels just dont look right to me on a 250 GTO
Cheers