Advice Needed - Can't Start Rebuilt 273

I had the same problem with the 360 Magnum I installed in my 74. It would start and the shoot a big flame threw the carb. Tried everything. Checked firing order, checked for spark, and even checked if the distributor was aligned correctly. I had a bought a used Edelbrock 750 for the engine. When I received the carb, I rebuilt it. The engine seemed to get plenty of fuel. I could see the carb shooting fuel out of the primaries. It would keep starting and then backfire threw the carb. My son keeper putting out the fire with a towel LOL. My friend said that we had tried everything and checked everything except the carb. He said that to get a squirt bottle and put some gas in it. We removed the carb and poured fuel into the intake. The engine started up and revved up to around 3000 RPMs before quitting. I bought a new Quickfuel 750 carb and tried again. I had my son put the carb on. This is his first car project. He is 15. When we tried again, the engine would start and backfire again!!!!! It turned out that this time my son had mounted the carb on top of flat washers. I didn't notice until I started to remove the carb. The car runs great. I would double check the carb. Hope this helps.