The Secret Society of Lathe Trolls

I have a rebuilt Presto K8 and a rebuilt Presto K8 1D 500ohm head. The K8 has a 8ohm output to the cutter head for the stock 5c. I’d like to use a transformer to use my 1D. Do I need a 8ohm/500ohm transformer or do I need a 500ohm/500ohm transformer.