Hi Alex,
I decided to do this to the tank after seeing the excellent work Super Dave did when salvaging/restoring the tanks for his carrier
My first carrier: some assembly required, post176
After draining the very old yellow stale gas from the tank, I added a few liters of hot soapy water and sloshed the tank around to remove any remaining fuel. I repeated that process two times. The soap I used was a concentrated degreaser type of soap. I then left a fan blowing fresh air through the tank overnight. I couldn't smell any fuel the following day.
To cut the tank, I used a 3/64" cutting disk on an angle grinder. The thin disk leaves lust the right gap to have a full penetration weld when you put it back together. The wall thickness of this tank was 16ga.