I read your original post as well. I recommend you return them and then get the standard length. I doubt you could have them tailored and more importantly, if you did, it may compromise the waterproof-ness of the pants. Better to have what you want that way.
I have a pair of the XP rain pants and like you, I'm 6'5". I have the XXLs, since my waist is a bit bigger, but need the inseam length as well. Mine are the standard length.
I've found that FJ pants tend to fit on the longer side, which I appreciate. I'm a 33" inseam and I can generally only fine 32"s when shopping, where 32"s feel short and 34"s feel too long. I tried on a pair of FJ performance pants in 32" and they fit longer than some of my custom tailored 33" dress pants.
Hope this helps and best of luck in finding your rain pants in the proper length.