Make sure your distributor cap is the correct one, and isn't broken. My dad and I replaced the one on the Vette last summer and it kept backfiring we couldn't figure out what was wrong. We forgot the order but we were able to locate the correct one on the net. But it still was back firing. I was playing around with it and was checking it out and there was a peice on the bottom that wasn't allowing a full connection, which was causing that. Try putting your old one on and see if it works in that order if it does then it's your cap. I just hope you kept your old one.
§am.