I’m not sure which car you have. I’m still a student so I get AutoCAD as part of my tuition. I believe in AutoCAD you can make cuts in the assembly. Another option might be to save the assembly as one part which I think you can do and edit it from there but then you can’t go back to editing each part.
Unfortunately if you want to keep the pieces separate, the best bet might be to sketch it in the assembly and measure the relevant parts in the assebly to change the cuts in each piece.