Again: Unleaded or leaded fuel for mid 70's /6?

100LL Avgas for racing is fine if thats what the engine likes.
Avgas for the road is not allowed. If you are using it, keep your trap shut or you will lose your supplier. While the EPA may not sniff you out, the revenuers are out looking for shanningans like that.
Race fuels are pretty much the same situation, regardless of the TEL content. Occassionally there are exceptions but its rare.
This why you can fill fuel containers but not pump right into the fuel tank - unless you're at a race track.