This is what the jan 71 240z with fiberglass rear quarters came with:

Should the black filler flange be on the back side of the metal (like on my 280z)?

This the correct location of the filler flange for 240.

Should the black filler flange be on the back side of the metal (like on my 280z)?

No. My unmolested '71 was assembled exactly like that, and my replacement fuel filler pipe is like that now. The early 240Zs have captive nuts on the underside of the fuel pocket, so there's no other way to effect a seal here.

Thanks Again!