poping noise from exhaust

[frankyjoet]

On my 73 350 GMC man tran ,timing set between 10 and 12 btdc . have rv cam, roller rockers dual timing chain, i have an after market dual exhaust , i don't know their size, the whole exhaust system welded. have 2 magniflow mufflers. at first i thought the timing was off, but after searching with google, it seems the exhaust system is at fault. what is the timing set with a RV cam, also have ceramic headers edlebrk 650 4 bl with an edlebrock intake. need help thanks for any help