I cleaned my 56 gmc tank out 10 years ago- what a job! I used a a lot of 1" stone, probably same effect as chain. The acid flushing is probably toxic waste, what with all those years of lead. I remember it took a lot of muscleing the tank around.

Well, my time went so quickly,
I went lickety-splitly
Out to my ol' 55.
As I pulled away slowly,
Feelin' so holy,
God knows, I was feelin' alive..... Tom Waits, Ol' 55