If you completely drain the tank then put enough PB Blaster in the tank to completely cover the threads, it will be much easier to break the plug loose. Let it soak for a while before you try to turn it.
Yes, I saw the tank do the squiggle like Boss
I am aware of the banjos available but for me, even that banjo was uncomfortably low.
I just put in a modern 22mm plug and went the traditional route with one exception, I put in a new combination float/pick up, calibrated the float.
My tank had the early original integral fuel pick up which had rotted through about 12" up from the bottom of the tube.
I can't speak to the factory tube capacity for running at full throttle, flat out in an endurance race situation but so far with the Webers there has not been any kind of a full delivery problem for me.
Now, if I want to go with FI, the original fuel pick up supply tube can be used as the return and if need be, an internal pump attached to the removable pick up/float sensor assembly can be added and altered to any number of "production" in tank pumps.
I am using duel external pumps. A Holley electric pump and a Holley high capacity mechanical pump. The electric pump primes the fuel system after a long lay off. My system is reduced to 4psi at the carbs so all things are not equal between carbs and FI.
IF you decide to feed from the bottom of the tank you will NEED a positive shut off as close to the tank as you can get it.
Strange things can AND DO happen. There is no possible way to predict what road debris can do. By definition, unpredictable is unpredictable but the last thing that you want to happen is that you need to get under the car to shut off the flow of fuel from the bottom pickup in an emergency situation.
These manual shut off valves also have a nasty tendency of not working when you need them too. Virtually freezing open not to mention the O-rings drying up and eventually leaking. They are just another component to fail on you. Add more components, increase the mathematical probability of a component failure. Simpler is better...and safer.
In that sense the factory type of tank pick up is much safer. As a result, I feel that in this car, all fuel lines, pumps, filters, etc, should be kept ABOVE the level of chassis. Nothing should be below the bottom of the tank.
My car reflects this point of view.
As far as loosening the original tank plug,you could just take a torch and put a lot of heat on the fitting instead of PB Blaster?