I don't know the 90 fuel tanks at all - but it looks a lot like my aux fuel thank on the 110 under the passenger seat. On mine the filler comes off and my arm fits inside, or the breather/fuel return fitting comes off and you could turn the tank upside down.
Or am I guessing it's fitted and you don't want to remove it again?!?
You going from V8 to 200 TDI? I think I have located a new dash from a donor 110 sitting round the corner here. Think it's been dumped, I'm watching it closely. I'll get my dash totally off, strip back all the left in wiring and put the 'new' dash on. Can't wait to get rid of all the old wiring that seems to go no where!!
Really jealous watching you put this thing together, would love to strip mine down and rebuild it without the mess - if I get mine back to UK you can help me move the steering wheel over to the other side.