I'd probably leave it alone and tune on it. I found that my old 440 w/ the 528 liked a tighter lash. I went form the factory .028/.030 down to .024/.026 and it seemed to pull longer. The 850 is probably your best bet for a carb.
The 516 probably don't flow as well, but they do give you more compression. If you do use the other heads at least do a gasket match home porting job before you drop them on.
I'd think the 413 w/ some tuning would be at the 12.50 mark. The 509 would do it easier. But the 509 in the stock 400 would be a total dog.