@273 said you could do the logs now and upgrade to good headers later. That is good advice IF the B body logs will fit in an A body, and I am not so sure they will. Evidently,
@Carnut68 is not sure if they will fit either. That is why the A body big block logs have different part numbers than B body logs. You need to find out whether they will fit or not before making a decision. Please do not even consider using the fenderwell headers.
If the B body logs fit, I would go that route for now with the intent to upgrade later.