Heater Box Question??

Attached is a heater box and my question is:

Is the fresh air box necessary if the a/c components have been removed?

I only changed the heater core and removed the a/c condenser.

Can I block the area where the fresh air box attaches and still have a functioning heater? (Note: there is an intake from the box to the upper portion of the cowl, I am presuming this is for the a/c only)

Any advice would be appreciated.