80A is what the original strip fuse on the bottom of the stock GP relay is rated for. Seems like that certainly should do the job.
IIRC the theoretical math is the "fast" glow plugs like these draw around 12A apiece once running at steady state. So if that is the case then with 6 live plugs 72A is the current the system needs to carry. 80A would give enough headroom to accommodate that with a good margin.
The fun use of those figures is when there are symptoms of a dead plug or two you can measure the total current to the glow bus with an inductive ammeter and with that number determine how many plugs are running and how many are dead. Of course then after the science experiment the task of finding the dead plug(s) still remains.
