I ordered 15 x 8, -8 offset, C8's in the late 70's.
When I received them I had to send them to a machine shop and have the centers enlarged to fit my 73Z.
You might try Classic MotorSports in Santa Rosa.
Ken Anderson has tuned my my Z in the past. He is very good with mikunis and fuel injection. Years ago I believe he did fuel injection tuning for Dave Rebello?