Anyway, back on topic, it is worth looking into what cable you choose to use, as you can struggle with voltage drop through running an extra 5m of cable or so.
I have relocated the battery on the last two 172's that I have had, put the original full size battery into the wheel well. I kept the original battery as I don't use the car much and was worried about starting performance after the car has stood for a week or so.
Starting is definitely more sluggish, and on the dash binnacle with the ignition key switched to two, instruments and fuel gauge would show as normal, and fuel level displayed. When I would start the car, the fuel level indicator drops to zero, before then returning to the correct setting. This did not occur when I had the battery in the original engine mounted position. Once started the car is fine, and with a multimeter it was running about 14.4V at the old engine positive terminal, which is about right, and suggests that the only issue was on start up.
Regarding earthing points, I have used existing earth points in the boot which are used with the rear light clusters.
I have also had a battery fitted under the passenger seat before I then fitted buckets, I earthed the car by fixing to a bolt that I put through the front transverse rail that the seats the mount across. As I was only using about two metres of cable, start up was a lot better from having the battery in that position.
I was using 25mm cable, and in hindsight should have got better quality.