I just rebuilt a 1983 5.0 and it won't start or even pop. I have the summit 1790 cam, GT 40P heads. I have had two different firing orders given to me. I called summit and they told me 15426378 which is different from the original firing order. I'm at my last nerve. I have no idea what to do. I've checked the timing, spark, fuel and it's all good. I'm so lost.