Ok. Front gears have to be the exact same ratio as the rear. Have to swap both ends at the same time.
Gear swaps needs to be done by an experienced tech.
There is another way tho. Ford made 410 axles for these rigs. You can simply swap the complete axle assemblies.
Gear ratio codes are on the door sticker, and tags on the axles. You can decipher the codes here, in the trans differential sub forum. Once you know the proper code, you can start searching for the axles.