Hi WWC
The pump does have to sit inside the tank and at 6 inches, the lid is not big enough for it to fit in. The simple solution that most people use is to cut two slots through the tank, starting on the edge of the lid and going out towards the edge of the tank for about 6 inches. This will allow you to fold the flap open, push the sub pump and hose in, then fold the flap back down and glue or seal into place. The hose and cable can often then be feed through the hole in the centre of the lid (on most lids) and the lid can be screwed back into place.