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.

Oh, sorry to include some very important information.
Running Pertronix HEI Distributor and 8m plug wires

Open headers will sound funky, wait till you get the full exhaust on and then diagnose if needed.

Yeah, it seems ok for a bit, then pap, then ok then pap, I will see if I can get the video uploaded.

It also seems to only be coming from the drivers side.

Double check the timing.

I had my fresh 455 running with open headers in the garage several times before I got my exhaust installed and I did not have any backfiring problems. The engine was broken in on a dyno where they zeroed in the timing before I installed it in the car.

Did you run a dedicated 12volt wire to the new ignition?

I installed a new autowire kit for the car, it has the stock wiring setup, so I assume it is correct. I plan on checking the plugs, and wires when I get home, as I have heard that could be an issue as well. Wish I could upload my 26MB file somewhere, hearing it might help.

Use one of those freeware video conversion programs to downgrade it....

Check #5 and #7 plug wires; make sure you have them in the right order. Good trick to play on someone.

Ok it was not a real backfire, it appears that my engine ground strap got cut somewhere in the process, and it was actually arcing to the frame. New straps have been installed, but I think that it fried my gauge senders, as both oil, and temp jump up to almost max when the key gets turned on. So I need to get that worked out before I try starting her up again, need to make sure she does not get to hot, and there is good oil pressure

I lost one of my gauges but I think it's going to need a new board.

You may have the same issue.

Well after all that, worked on it this weekend, and it turned out to be a tiny crack in one of the spark plugs, hard to see except when it was really dark. Runs like a top now, but since this was the first good startup, and she runs so good, I have to go back and address the various leaks, ahaha. I am so excited, so thanks again to everyone as you have been so much help.

Nothing like a fresh motor. I can not wait to get mine in the LeMans. Just having that power under your right foot is such a great feeling! Glad you solved your backfire issue!