I'm not sure exaclty because the fitting test can't go too far without cutting the resin hull first: anyway I had to unscrew the whole back of the metal hull, and it will also be necessary to remove the front metal hooks and front plate, and possibly to cut a bit of the inner aluminium front plate (maybe this is not needed).
As for the resin hull, the rear requires at least the outlined part in the picture below to be cut (is is meant to slide inside the hull) while the rest of the hull follows the rear rounded wall, the one around the final transmission cover.

But since I will be using the metal hull for the Su-152 and have ordered a plastic one for the JS-3, I'm sorry but cannot go more far in my hints, and I'm not sure about how the resin hull will copy the metal one, even after these works. It should, though.


Uploaded the picture twice since the first version wasn't clear. See the second one with the dotted outline.