In my many years as a designer I can strongly recommend a benchmarking phase to any undertaking.
You are designing a skyscraper? Look at ten and see what you like, what you are aiming for. Symmetrical, asymmetrical, stepped back, shaft like, horizontally banded, vertically banded, monolithic etc etc. Pick a beautiful coupe that has been "stretched" that you have seen in person
and use it as your go-by. If you come up with approximately zero examples you have two choices:
1. cancel project.
2. understand that your 1000000 hour project will be, and look like, a prototype when you are done. if that's what you want, have at.
Another recommendation is to question the presumptions. Is the goal really to "do a big project" - which I can respect. I like a good project. Or is the goal to make a 'high end gt' sort of like a cruising version of Alfaholics GTAr? If the former, does it absolutely need a straight six (why not V?) or twelve (why not 8?) cylinder motor? If modern high end GTs are your benchmark, cylinder count is going down across the board and packaging is getting more efficient. If the big motor is determined to be a must, are you positive there aren't lots of options that will fit in the stock bay? I'd sooner do surgery to say, the cross member or firewall than I would stretch the exterior envelope of the car.
I spend a decent amount of time idly pondering engine swaps to my E30. I was never attracted by the heavier E36 M3 motors that are so popular but recently a lot of capable people have spent time making late model all alum/magnesium straight sixes swappable with no cutting. To me this is a cool thing - keep the low displacement, torquey, sweet sounding straight six but use the latest and greatest from the same family, adding not just power but lightness and efficiency. With no cutting. That is cool. Same basic idea as the Alfaholics GTAr in fact. Which I presume you have seen? https://www.alfaholics.com/gta-r/