when i worked at what was R.O.F. Barnbow Leeds a long time ago (now Vickers) we made the Chieftain and then challenger one
most of welding was stick front glassis plate was 2or3inch thick they used 1/2 inch rods and 600amps maybe more mig was used more as rework on parts for rectification work i used to work in the stores part of the job was filling the ovens with rods and changing the oxy acet bottles over
http://en.wikipedia.org/wiki/Chieftain_tank
ad he a son called keith cockroft??Did you know a little short bloke called Arthur Cockroft by any chance....?