Fox, I can see where you'd think that the flamer box is a different size, but it's all in the angle of the pic you posted the link to. They are indeed the same width and height.
When I began my quest to build my Jango, I started by comparing all of Bobas gear to Jangos. The main reason for this was I wanted to know if I could simply convert one to the other, due to the readily available Boba parts in comparison to Jango parts... which were far more difficult to come by at the time. So I know full well all of the subtle differences between Jango and Boba, from the sharper lines on his gauntlets to the 5mm height difference in the helmet domes.
So yes, you can indeed convert a set of Boba RoTJ gauntlets to Jangos as Fox has stated. The best part is that when it comes time to alter the flame thrower box, aside from shortening it, you'll only have to scratch build the front portion that houses the flamer tips ( talk to Cruzer about the flamer tips ).