them 8n jubilee 1953 tractors are so rare.. in fact. so rare exactly 0 were ever made.. so kinda hard to give you much advice!!
both would be 6v +grnd.
8v bats are only a source of problems, as the generator/regulator is set to charge a 6v battery.. thus leaving you with a near dead 8v battery soon.
better to fix the 6v system or move to 12 vs trying to go 8
tell us which tractor you have.. an 8n or a naa and we can get to generator testing. there was a field circuit setup change in the charge system between those 2 models .. they test and polarize differently