340 and octane

I have a 340 in my dart. It has home ported speedmaster heads with 2316 pistons with 9.86 comp. and .035 quinch. Cam is hughes solid 237 and 242 at .050 and .537-.543 lift on a 107. Induction is eddy street tunnel ram and 2 69 340 avs carbs. Anyway, I always run 91 octane but cant buy it in my home town so sometimes I run 87 and baby it to the next town to buy fuel. So I had 87 in it and i never get on it because of fear of detonation. Well I forgot this mourning and hammered on it. Car felt way stronger with no audible detonation. Timing is at 34 total. Do you think its safe to run 87 and should i check plugs for detonation. Does this sound right or am i missing something.