1947 Gas Tank (help needed)

I need to find the right tap to chase the thread in the gas valve hole.
I thought it was 3/8 X 18" NPT but that tap was too small...

Anyone ?

Locate an old gas valve. File or mill slits in the side of the threads and use it as a thread chaser.

Nicke, if chris's method doesn't work cause threads are really boogered up, the tap you need is a 3/4-18 NS. Mine was a special order.

Thanx for the help guys!