In the mid ‘70’s, USA was going to unleaded fuel. Catalytic converters will get stopped up (and oxygen sensors) using the cheaper leaded fuel. There might have even been health related items as well using leaded fuel. I will attempt not to go into a rant about the whole issue - for I have some disagreements with issue. To prevent people from using leaded gasoline, the unleaded fuel filler nozzles had a smaller tube – which fit into the “flapped” hole in the cars fuel filler neck.
The cars that were not for unleaded fuel (mostly for export), did not have cat converters on them, had a full-size fuel filler neck. In the USA, leaded fuel was banned for sale starting in 1996 (and in NZ the same year, it appears).
Sense leaded fuel is no longer for sale, there is no need for that flap or flap plate in the fuel filler neck – so if you want to part with it, you could cut it out (I suppose).
Getting back to your original question, if a person was to find a second filler neck, a person could cut off a portion of it and weld two filler necks together to get the length you need. Personally, I would use the fuller neck hose to lengthen your metal tube with. The fuel tank is not under much pressure so I would not have a problem using hose.
Now to your last question, hum, while scratching his hairy chin. I don’t recall seeing a lack of a rib, there before.
Before committing to the fuel tank move/change, there are a few options available to you.
One is use of AAR/TA Challenger side exhaust
Another is to run the drivers side exhaust over and above the differential and run both out the passenger side of fuel tank. It would still be dual exhaust but exiting out one side. I think Aspen500 has this setup.
Even another option is to run exhaust between the leaf spring and behind the rear tires. I have seen pictures of that option in our forums, as well.
I hadn’t gotten that far along yet to have any recommendations. I really like the AAR/TA size exhaust – but that is not practical on more-door cars for it tends to burn rear passenger legs on entry/exit (this is not a problem on 2-door cars).
BudW
Pictures of AAR/TA exhaust system