I'm a little confused by your question. The '47's didn't have a firewall fuse box. The only fuse shown in the wiring diagram is mounted on the light switch.

How many fuses do you have in your firewall box?

The '48 and '49 trucks had a 5 slot fuse panel. All the fuses were 20 amps.

The 50 and up AD's used a 2 position fuse panel for stop and tail lights, both 20 amp fuses.

Hope that helps.