Depends... Ford had different gear materials for different engines (ie roller vs. mechanical). Regardless, you never EVER put new gears in with old ones. This goes for axle gearing, engine gears (diesels), distributors , etc. After a TON of miles racked up, the gears wear in together making them a matched set. There's no better way to destroy a perfectly good gear and littering your oil system with metal than to install old on new.
1978 CJ7: 304, 3Spd, AMC 20 Rear, Thrush Welded Mufflers W/ Turndowns, Unknown Front, Worn out Dana 20 case