Fill it through the top (uniflow tube). Bottom tube is the overflow. During flight it is normally capped off. 

Correct, only the overflow is capped off. The top (uniflow vent) can either be vented to the outside (cap left off) or attached via a piece of tubing to a fitting on the muffler. That is what is referred to as muffler pressure. Some engine/set-ups (and fliers) like it, some don't. Try it both ways and see which way you prefer. You will have to adjust the needle valve when switching from one to the other. Leaner on muffler pressure if I remember correctly.These are the set-ups most fliers use with uniflow tanks but there are others...

If you block off the uniflow vent and leave the overflow uncapped, you will have just turned your uniflow tank into an an old style suction tank. Not to my liking. You can also keep the uniflow blocked and attach a muffler pressure line to the overflow. This will pressurize the tank and can work OK.

As a starting point, just block the overflow, keep the uniflow vented to the outside, and go fly. You can experiment with these other variations if you have trouble getting a consistent run with the first two set-ups. 

Thanks Pete,

      Now I just have to get to the field and try.  I don't currently have a muffler set up for pressure.   The overflow tube is forward facing.

    Thanks again.

I'm sad to report.  I got one half decent flight in but didn't have the needle right.  It was running rich.  Then I switched lines from 52 ft to 70 ft.  We thought it was 60 ft.  Well, it was running great but hit a pole with the outside wing tip and buckled the plane.  I don't know if we'll try to reconstruct the cranky Nobler, as I had it on another post in Open Forum.  I was getting it to fly better with the suggestions made there about trim.

The B-40 and the tank are now in a Smoothie, awaiting a new first flight in that plane.

Life is interesting!