Popping sound in exhaust

Thanks for the replies! It definitely sounds like its lighting off in the headers. You can hear in the tail pipes pretty loud. It does it right from the get go after starting before the headers have time to heat up. I've checked the ignition including firing order. Not sure about the rotor phasing yet but I'm thinking the problem is the exhaust valves hanging. I bought the motor complete but apart. The heads had been done by a machine shop and were still wrapped in plastic. Don't know what shop, though. What is the correct length pushrods for a stock 1990 360?