The No.1 Express has a shoulder so you can establish your proper head spacing off of that.

I was doing the same when I got my first No.1 Express - using 500NE brass.
Gradually adjust your die so that the action closes snugly.
The little space between the rim and the extractor is not an issue as the web of the NE brass is so stout.
It’s funny, the 500/450 No.2 Musket is easily made from 50/90 Sharps brass which Starline makes at a very good price. I inquired about supplying the same cases long enough to make No.1 cases and they said they couldn’t do it. Their extrusion machines must have a length limit or something. Sure would be nice though…