As with all Frankenstein guns, there are always fitment issues. Luckily the Katana GB isn't too bad. I'll post pictures later today of what I did to get this to fit.
To answer your questions, the lower gearbox fit like any other V2 gearbox. The only difference being that you do not have the upper portion and you must grind down two small tabs on the top of the gearbox in order to get everything to fit properly. This only took about 3 minutes of grinding with my Dremel. In order to get the cylinder to fit inside the upper, you must grind down the tabs on the side of the cylinder and the tab