I cut the channel on a big cold saw so the ends were bang on, when I welded it I clamped some plate to get the top flanges as flat as possible.
You're right though "parallel flange channel" isn't really parallel.
I used 20 odd metres of 6" pfc, about 12' by about 6' and waist height.
The floor isn't quite flat, I made adjustable feet to get it level.