I'm not a car body expert..but cut n shut will be a very difficult and high precision task.
Don't do it until you or the body shop is equipped with and expert in making limousines.
hard top comes down with MM precision.
I think cut/paste of the rear portion should be a better option. Again, im not an expert.
Btw...Soldering is done on electronics, jewellery and plumbing....for car it would be welding !!!