Causes of detonation?

I got my Carter AFB 625 cfm installed on my 318 in my '70 Duster and I'm having some weird pinging issues. Before I continue it's an A904, 2.76:1 open 7-1/4" car and the 318 has stock bottom end with reman'd stock #302 heads, stock exhaust manifolds and single exhaust, Lunati Voodoo 256/262 cam, Edelbrock Performer intake and of course the Carter AFB, all running at 6500 ft. elevation. I also use 85-octane gas.

What's happening is when I accelerate with just the primaries it's completely normal, but when I get into the secondaries it pulls harder up to about 3700 RPM then starts to "lay down". At the same time I can hear some light pinging coming from the engine and I usually back off before 4300 RPM. At first I thought the secondaries were too lean, so I drilled the jets (.086" size) out to .094" but the pinging is still there (pulls a little harder though, haha). I'm going to try checking the total advance this weekend but I'm pretty sure it's not more than 36* (of course I'm not sure what RPM that's at though).

One thing I've been wondering, can you get detonation from an overly restricted exhaust? The stock system can't be over 2" ID (on a single pipe, THE HORROR!) and has multiple dents in it along with a very quiet Sears "Muzzler" muffler (what the car came with, the name kind of scares me). Could excessive backpressure be causing this detonation? I'm 95% sure it's the reason why it won't make power over 4000 RPM even with the cam and 4-bbl.