It was a bit of a stupid rule, the registration only required the build date of the conversion engine to be of same year or later than the vehicle being fitted to. You were only required to use unleaded if the ADRs on the vehicles factory compliance stated so. Of course you cant buy leaded fuel anymore so you don't have an option. If the heads on the SBC engine you have doesn't have hardened seats for unleaded fuel you will have to use a fuel additive or the valve seats will only last 10,000 km or less. Most SBC engines by the mid 80s had hardened seats. Again get the casting numbers of the heads and do a search. The only other way is to remove the heads and inspect.

If you have the old registration papers have a look and see if the SBC engine number is on the papers. This can be proof that the engine had compliance when it was installed and should make it easy for the blue slip guy.