Headers for 73 Duster with 451

I called TTI and I asked about different headers they have. For my application which is a "B" engine with TFS heads they said the ones that would fit nicely without any modifications are the 400SR-218N.
Those are 2" that step up to 2 1/8". I did ask about the headers that Hotrod360Andy mentioned and the person said those wouldn't work well and would touch in a few places. They are designed to go with the 440 with those heads or B1. I would think and I did mention the fact the "B" is smaller and he agreed. It's just that he said they were meant for the "RB" . I asked about the 1 7/8 and he felt it might have been small for the 625 to 650 HP levels that I am at now.
I will order them in a few days. Anybody use step headers? Are they O.K for a street/ track car in those power levels? Any Benefits to the straight 2"? Just curious