The 1st person mode you're talking about is just a 3rd person mode with camera moved to the player character's eyes level, thus it's very experimental and not recommended for standard gameplay.
But if you really want to have it, then follow this guide to extract game's files from the .DAT archives and then look up this topic to know what you should modify in those unpacked files.
PS. If you can't access values in the FPP camera topic, then install uBlock Origin and add this custom filter to it:
rockraidersunited.com##.ipsEmbeddedVideo