Call me crazy but I like my longtube headers. A 1 3/4 long tube should make gobs of low and mid range torque for a long legged 400. I just pulled the tri y's off the 440 in my duster. I find it hard to believe they would outperform long tube headers.
I doubt the tri-y's,would be as good as the right set of 4 into 1 long tubes. But I do think they would give better very-low end torque, (that's what tri-y's are supposed to do) and be much easier to fit.
Besides, where you gonna get c-body headers? Try to fit b-bodies?