Water pump/ radiator questions

[36coupe]

I have a pair of skip’s pumps on my 81A.
Why is there a large hole in the block under the impeller?

I plan to order a 4psi check from Skip. Will my stock 36 cap be okay or do I need something different

 

[rockfla]

Purely a SWAG here but I would imagine it was left in the casting for use in "earlier" V8 wanting to use "lower outlets" with "in the head" water pumps and a "blank off plate" for where the later style water pumps go??