I see why they sized you at a 12 then. Usually you wanna try and size to your heel to ball measurement so the arch support fits in the right spot. With your toes needing so much room, I would normally say the 38 swing last might be the way to go. However, since you have high arches, I don’t know if they would have enough arch support for you…well, I guess if the Lower arch support in the MP works for you, the 38 should.Yeah, as I've gotten older I've come to just embrace my lanky build and long feet. I'm 38 6'5" and 195lbs so not exactly a power lifter build ;-)
The 12 MP's have quite a bit if length. I wear Costco merino wool socks in them and my toes don't hit the end even when I crouch down on the balls of my feet. Definitely no extra toe wiggle room though.
My issue is that my Heel to Ball of foot size is more like an 11, but my toes are a mile long...
I feel your pain man. My feet are similar to yours in that I have a narrow foot, even narrower heel, normal arch height, but low instep. However, I have short toes. My feet measure a 13C, heel to toe, but my heel to ball is a 14. I’m 6’5”, 320lbs. All my Whites are on the 38 last, in a 13C with an instep adjustment for my low instep. 3 of my 4 pairs are combo last, 13C with a B heel and they fit perfect. My problem has always been that if a boot fits my forefoot good, the heel is loose (major heel slip), and the sides of the vamp are almost touching with the laces pulled tight because of my low instep. If I got one to fit my instep and heel good, then it was too tight around the ball of my foot.