I would use liquid mask to cut the border around the windows, the finished result will be much better this way
Don't be scared of the liquid mask, you can always cut up a old 2/3 litre coke bottle, wash it, stick some mask on & practice your cutting on that, use a new decent scalpal & you will soon get it right
I used to have no confidence in liquid mask but as soon as i started using liquid mask from RCS i haven't looked back, just follow the guide on Jon's website
http://www.rcsgraphicworx.com/
If you put this stuff on properly you wont get any paint bleed & your new shell will look great
If your putting mask on the outside as well, draw your window lines on the outside, then you have a line to follow when cutting