I have basically the same issue with a really stout battery and insulated mini starter. It cranks fine but takes a bit to fire. Went over everything twice and came to the conclusion the real problem is the **** gas being produced these days. Today's fuel is blended for closed systems using injectors and electric fuel pumps. To prove my suspicions, I bought and added octane booster and the issue disappeared. The newer fuels seem to evaporate quicker leaving fuel bowls mostly empty and it takes time for mechanical fuel pumps to replenish the fuel bowls hence a lot of cranking.I know 383's suffer from "hot soak" issues even in a massive engine bay but the cars of yesterday never seemed to have this problem, but then again, fuel of yesterday was heavily laced with lead. Bring back leaded fuel - it's not like people are drinking it. Methonal is something that should only be used in camping lanterns.