Someone school me on header size.

As a basic description, but not limited to it, header sizes can vary greatly on any engine and have little effect. There is a point of cost vs. return and diminishing returns.

1-5/8 - Stock to mild street strip.
1-3/4 - mild street strip to medium strip
1-7/8 This and larger header primary size from here are pretty much for a strip car on a small block.

Cubic inch displacement should also be considered. If you have a stroker, every tube size description moves one spot. The 1-3/4 move downward to mild street strip because of the amount of exhaust your moving and how the engine reacts vs. the displacement.

Collecter size is designed into the header from the manufacturer based on its experience in this field so at one point, they said this header made better power with a 3-1/2 inch instead of a 3 inch.
Same issue with tube size. But that and all the parameters of a header can be constantly change for what there intended for and debated endlessly.

TO ANSWER THE TTI QUESTION

The step header is a bridge between the two tube sizes of a single tube header.
TTI does (or did?) rate the header for best area of HP output. It would s labeled good for. Example; 575 HP, next header, good for 750 HP

These are general guidelines. You can use them for more or less power with certain draw backs outside of there general recommendations.

IMO, it would be an excellent choice to use. Hedman makes a solid single size as well as a stepped header in various tube sizes. I’m not sure if they have what your looking for, but I would cruise the site and browse around there long list of headers to see if they have what you or your engine builder like.