Installation:

1: Install this mod:
https://www.gta5-mods.com/vehicles/supermarine-spitfire-mk-iib 

2: Add the "spit_sign_6.dds" file to "C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto V\update\x64\dlcpacks\spitfire\dlc.rpf\x64\levels\gta5\vehicles\vehicles.rpf\spitfire.ytd"

Credit for the Supermarine Spitfire Mk II goes to SkylineGTRFreak