I just installed the same overflow so I think I know what you've done. That said, here is what I recommend:
Make a spacer plate (with the necessary matching holes) out of hard plastic as well as two more rubber gaskets. Your installation sequence is:
- internal overflow box
- gasket
- new spacer plate
- gasket
- tank wall
- gasket
- external overflow box
- nut
And, if you are still assembling and haven't installed a gated valve on one of the 3 drain lines, do it now or you will find you have a very noisy system. Lastly, the short, red overflow pipe is standard 3/4" PVC so you can tweak how the overflow box works my fabricating slightly different lengths. You will probably also need 2 PVC 90's to redirect the main syphon suction. Cut the 2nd 90 in order to keep the opening relatively close to the bottom of the overflow.
Make a spacer plate (with the necessary matching holes) out of hard plastic as well as two more rubber gaskets. Your installation sequence is:
- internal overflow box
- gasket
- new spacer plate
- gasket
- tank wall
- gasket
- external overflow box
- nut
And, if you are still assembling and haven't installed a gated valve on one of the 3 drain lines, do it now or you will find you have a very noisy system. Lastly, the short, red overflow pipe is standard 3/4" PVC so you can tweak how the overflow box works my fabricating slightly different lengths. You will probably also need 2 PVC 90's to redirect the main syphon suction. Cut the 2nd 90 in order to keep the opening relatively close to the bottom of the overflow.