Manifold Flange & Gasket Issue

What is your question?

I just installed dual catalytic converters on my truck. On the passenger-side, I could not get the flange that slides on the pipe to fully cover the gasket and meet the flange on the manifold, in order to completely conceal the gasket and ensure a tight seal around it. I want to know if that would cause the truck to not start? It cranks but wont turn over. Every once in a while it would start and shut right off. Other than the fuel pump, regulator, relay and other emmission-related issue possibilities, could that be the problem?