Instead of cutting your tank apart, can you fabricate a sending unit that inserts in through the filler neck? If there is room, maybe the filler hose could clamp to a whole new assembly that inserts into the tank (like a "Tee") and threads into place using the original threads intended for the fuel cap?
I love seeing that you built this rod from scratch! A little off-topin, but can I ask how you made the belt line? After looking at the first pic, I wondered how you added it...?