70 RA III is 10.5:1 CR if stock - so you should burn the highest octane pump gas you can.
With pinging/knocking are you sure it has the correct timing? Might be too far advanced.
Lots of good advice on this site to help you.
is correct. If factory 10.5:1 compression, 93 octane will not cure the detonation. You need to get up around 100 octane either through racing gas or an additive.
You can try the colder heat range, but it won't cure detonation where octane is the requirement. Cooler plugs may be something to try - it can work to a point, but they can also foul easier if doing easy driving or city driving. So if you don't like to "exercise" the engine, you may gain nothing.
Adjusting your timing can be a bandaide of sorts by backing the timing down - retarding it. You will lose horsepower and if you begin to go too much, the engine could begin to run hot on you.
Detonation/pinging can damage/destroy the engine very quickly if you do not immediately back out of the throttle as soon as you hear it. However, you can still have this taking place and not hear it.
For this reason, most people when building an engine with iron heads will use dished pistons during the rebuild to drop the compression down closer to 9.0-9.3:1 or go with a larger CC chamber head - unless you are using aluminum heads.
You also want to get your engine to run cool, so you want a very efficient cooling system that works properly. Heat is your enemy and more so with higher compressions. Highway gearing and the heavy weight of the GTO's also add to engine loading which can also contribute to engine detonation.
That said, my opinion, I would first try running the tank near dry and add 10 gallons of racing fuel and see what you have. Do a local web search of gas stations near you that offer 100 octane racing gas. Some local stations do offer this at the pump. It is about $7.00 a gallon last I knew, so not inexpensive.