Farmall H tractor 6 volt starting with 12 volt will be problem?

Don’t believe it will be a problem at all. Wiring for 6 volt is larger diameter than for 12 volt and volts is not what burns wiring or starter motors up High amperage does. The seller has obliviously already converted the tractor to operate on twelve volts otherwise you’d have a charging problem as opposed to a starting problem. So in reference to your question, won’t be a problem at all.