But your idea of hanging the tank from the crane is inspired, so we'll call that one a win.
That made it so much easier and more effective. I suspected I was going to hurt my back doing it manually and mounting it on the engine hoist was so much easier and more effective to the point I had to wear ear defenders as the noise was horrendous as the bolts slid back and forwards. Putting the blow gun down the tank sender hole resulted in a choking cloud of rust dust coming out the filler neck so it is definitely working.