Material Selection - Modification

my Generator radiator water sump gets dirty all the time and requires regular cleaning. the drain point uses a 3/4 inch pipe. i want to open it up at the side by 300mm X 300mm. the tank steel thickness is 5mm and i intend to use 3mm steel plate as a cover for the port. question is, what type of bolt, size and number do i require. what gasket type.