Get your ESSENTIAL MANUALS out and read. ALL FORD's (not just tractors) were 6V/POS GRN up til the 12V diesels. Many N's have been switched out to 12V/NEG GRN but if either system isn't done correctly, will cause non-starting issues. Just because you have a 6V battery, or a 12V battery for that matter, don't assume your wiring is set up correctly for that system.
CORRECT FORD 8N ELECTRICAL SYSTEMS - 6V & 12V:
8N 12V SWITCHOVER ON THE ANGLE (SIDE) MOUNT DISTRIBUTOR:
*PICTOGRAM courtesy of JMOR
OEM 8N WIRING(6V/POS GRN) AFTER S/N 8N-263844 WITH ANGLE (SIDE) MOUNT DISTRIBUTOR:
FORD 8N TRACTOR WIRING DIAGRAMS:
FORD 8N TRACTOR ESSENTIAL OWNER/OPERATOR/PARTS/SERVICE MANUALS: