I don't have the kit in question and, while some full-sized tanks have a flange running along the seam, a regular step sounds more like the locating pins are mis-aligned. You could either:
1. Crack the tank open, clean-up the gluing edges (removing any remains of the pins if there are any) and re-cement it for a better fit, or
2. Sand the seams on the tank as you have it, doing your best to keep it cylindrical. A Flexi-File or padded nail file is a good option for jobs like this.
I hope that's some help.
All the best