Humbucker sized P90s are your best bet.
I maybe wrong, but I think 250k ohm pots are preferred for single coils. However, that doesn’t mean your current 500k ones (I assume) won’t work. It seems that some folk like a combination of both https://forum.seymourduncan.com/showthread.php?256168-What-Pot-value-for-P-90-s-250k-or-500k&p=3387239