Oh dear, I just received an email from the guy at Kaufman's (I had asked him the same question, basically) and he said I was being 'fussy':
You're a tall thin dude and you're fussy about fabrics and you don't want anything 'cheap' and you'd like the flight coverall to fit comfortably, so your size in a genuine Nomex Air Force version would most likely be a 42L- period.
Stop over-thinking it.
Is it some cultural miscommunication going on? Was it such a weird question to ask? I had mentioned to him that, yes, of course, I would like a Nomex suit, but that I have to consider my budget limits; he suddenly seemed annoyed at that. I got the impression that I was suddenly a lot less interesting to him as a customer...
Anyway, enough ranting: would anyone have any answer to my question? Unless I'm not clear, in which case please tell me and I'll try to rephrase it more clearly...