You will spend some time on your back with your head up under the dash. The door fix is the most challenging part of a GTD build but it had been done successfully hundreds of times. MAKE SMALL CHANGES in one direction at a time and try and zero in on the best fit.
Some cars require a bit of material added on the corners of the spider to get a perfect fit but concentrate on the closing edge at the top of the door where it fits into the spider head hole. Once you have that good then the vertical gaps at the front and rear might may need to be fixed with some added fiberglass. Should you need to close a gap then add plys of fiberglass material onto the edge of the gap and when hard grind down the excess to flush with the door/body face.
Just take your time and work until your back begins to complain. Heed this warning, hobbies that hurt cease to be fun. Then stop and work on something else or have a beer and relax. This too will work out and your car will be beautiful. A lot of GTD builders have gone thru this and come away just fine. You however, will never look at a GT40 again the same way. I alway look at the door gaps first now, always will, I suspect.
Good luck, start a build log, we all like to watch and it will also get you all the help we can give.