Is it just ECU and alternator that need disconnecting? it's for a Granada. Cheers.

 

i usually take both the positive and negative lead of the battery and connect them together.

no problems then

 

just dis-connecting the earth would suffice! there's no need to join the earth and positive together after removing them from the battery terminals! you don't really have to disconnect any terminals if you keep the earth clamp close to the welded area as the current that you are using will inherintly take the shortest path back to earth. i know dozens of panel beaters in the trade that don't disconnect the terminals while mig welding.