I also just had a fume problem in my 1980 Datsun 1200 ute....such a bummer trying to fix it.

After much searching for the fuel stench in my cabin I found it was all the tuennau cover makers fault. The original clips along the side were a three holed clip, but I put a new tuennau cover on, which looks a lot better and this was just on screw in the middle of the eyelit. But when the first tawny was put on, he put the clips along the side of the tray and the screws were screwed in too far and it penetrated the liquid/fume separator on the drivers side, in the wall of the tray. Fumes from excess full were able to escape from these holes in the liquid fumes seperator and then travelled through the wall of my ute around the back of the cabin then in through the vents between the seats. so then i pulled the L/F tank out, drained it clean, welded the holes up, put liquid seal (Stag its called) on the hoses when joining them back up, and now she's all good mate! No more smell.

Hope this helps anyone in the future

Markus