To shorten the roof :-
Put the roof on the vehicle and make sure the window at the front is in the correct location.
This will give you some idea of where to cut.
Take out the back panel. You need to decide at this point if you wish to make the cut straight on the guttering or at 45 degrees.
Remove the strengtheners from the top of the roof using an angle grinder. These need to be overlapped when it’s put back together.
Shorten the roof by cutting straight round the corners and across the centre section.
Clamp the back panel back to the chopped roof and fold over the middle section. This can then be riveted to give it some strength.
The corners of the gutter can be welded back together.
This leaves the curved corners to be V cut and folded one by one. It usually takes a bit of cutting and filing to get it right. Once they are folded they can be blind riveted back to the roof.