Wow, you've thrown a lot of money at this problem, but it seems that each time you disturb the fuel system you solve the problem temporarily. Therein, I think, lies the clue we need to heed. New fuel tank aside, there may be crap in your fuel system which is getting stuck in the float valves.. You state that the car was completely restored. Did you replace all the fuel lines front to rear, hard and soft?
Also, modern fuel contains alchohol which degrades standard rubber fuel lines, breaking off tiny little chunks and sending them downstream. We now use nothing but fuel injection grade hose, which is formulated to hold up under these conditions. SOMETHING is getting stuck in your float valves.
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon