the earth on the head wont effect these values, its the battery, starter and alternator cabling which is what wants cleaning up and/or upgrading. mk3 golf will supply +ve cables that fit which are nice n chunky, -ve doesnt reach the earth terminal behind the battery but you could make up a nice chunky extension strap to fix this. Some B4 Passat models have a set of battery cables which are a direct fit onto a MK2 though, and are the chunky MK3 design.
you will see voltage dropping as you turn on all the lights, blower etc this is normal. can check mine later if you like to give you piece of mind