Have you ever tried those images with ceSky?
I do not use Bryce, so I have not tested its output with my program, but it should work.
I am sure the tutorials resulted in the six images you need, and the angles look ok.
If you need to swap images or flip them around then you may find they will fit right.
I had to rotate my up and down images 180 degrees on my first skybox that I made following a tutorial, but the images worked.
Anyway, ceSky is a FREE download at my website, and the instructions are in the zip, so feel free to try it.