400 SBC bored .40

Hey Fellas! Need help! I have a 1984 Oldsmobile Cutlass Supreme. I just dropped a 400 SBC into the car. On the first ever start up yesterday, it fired up and tthen died right away. It didnt sputter and then die as if it were a shortage of fuel. It seems like a ignition issue. Have plenty fuel gettin to the carb. Went to Autozone to get the control module tested. It tested out fine. I had the ignition module tested, that also tested out okay. I have also changed the ignition tumbler. It seems like the only time it will run steadily is when I turn and the keep the key fully turned to the start position with the starter engaged. The moment I let the key fall back to the on position, the engine dies as if their is no more power to the ignition. Some one told me to run my ignition to a toggle switch and to 12v and that will bypass and eliminate that issue, what do you guys think? I was also told that even though it is running like that it can still be 180 out therefore it would be a timing issue, but I thought it would back fire if it was that far off let alone run? I was also told to check valve lash? PLEASE HELP!!!