Causes of detonation?

Detonation is caused by premature ignition of the fuel air mixture in the cylinder. The cheapest fixes are retarding ignition timing or going to higher octane gasoline. You won't be getting detonation from an overly restrictive exhaust.

I have the 302 heads on a 68 318 in the Demon. With the stock heads, it was rated at 9.2:1. (Should be the same for your 70 model engine.) The 302 head has fewer cc, which bumps compression up higher. I'm estimating the engine has just a little over 9.3:1. I had a ping problem on 87 octane, but cleared it up with 89. The engine is too green to do a real hard run on it. Consequently, I can't say how it will behave over 3000 rpm.