Ok, today is the day that I finally got to start up my project. This is a freshly rebuilt 400 and it currently has straight headers, exhaust will be going on next week. My only concern was that the engine had a slight backfire, the other people helping me said the engine was running smooth, timing was on, and that the backfire was most likely due to lack of backpressure due to lack of exhaust. Does this sound reasonable? The backfire was not consistent, happened at random intervals.