Mixing ISO and JIS isnt advised since they are a different shape and that affects the connection between crank and bottom bracket. It can effectively push a crank out about 4mm due to the poor fit. And there is a chance of the crank breaking, blah blah yada yada.
Im not sure if the bottom bracket you are considering would work as I dont know jack about old square taper Campy crank arms. Are they anything like all other brands from that period and require different bottom bracket lengths depending on the spider and arm design?
Find out the chainline of your crank with a proper bottom bracket. Then buy a JIS bottom bracket that is a 4mm shorter. This will account for the poor fitment and place the chainline effectively where it should be.