You try wrapping a tape measure around it or across it.
If it was that simple I would not be asking the question.
I was hoping someone had a harmonic balancer in hand or on an engine on a stand.
You're apparently assuming all 302 harmonic balancers are the same. Without knowing exactly which version you have, or if it's even original 50+ years later, the best way to find out is measure the one you have.... As I stated I want to attach a "timing tape" to the harmonic balancer. These come on a card with @ 8 different timing tapes depending on the diameter of your harmonic balancer.
To index them to TDC, the tape you use has to be correct for the diameter of your harmonic balancer.
I was hoping someone had a harmonic balancer in hand or accessible to give me the diameter. ...