I had the same issue with my 77 620. Strong fuel smell even with the back slider shut. Pulled my tank and the return line was badly dry rotted. I replaced all hoses and hose clamps. No more problems. Also have a 720 4x4 that just jerked you out of your seat upon acceleration. Found that the feed line going to the fuel pump was dry rotted just enough to lose fuel pressure. Any Datsun I have had I always end up pulling the tank! :)