[F-BIRD'88]

You are brewing over and number that will not tell you much.

All that matters is it goes like hell and does not knock.
If you want to try a lower octane fuel to see if its enough, just do it.
Esso 92 or Pioneer 93. It will either rattle or it won't.
I can tell you neither are quite as good as the Ultra 94.
I have tested them all (local pump gas)
and this fuel makes a bit more power too. Key word: A BIT

If you need the detonation protection and like the slight performance gain:
Its worth the slight extra cost at the pump.
Pistons and rebuilds are expensive.

The R45TS plugs are a good city stop and go plug. "stock" heat range

The slightly cooler performance oriented RV8C (R42t) is a bit cooler heat range to give more detonation threshold
with lead foot WOT driving or towing where engine throttle load is higher. .035" gap gets it done very time.

I have been there done that as far as 305HO heads on a 350 goes...so....

If you really want to know the real cr you need to yank the heads off.