It's difficult to say because you haven't got rear door speakers or an amp so I am unsure of your wiring set up. You will need to get those door cards off first both back and front and try and see what you are working with and the loom at the HU. I would always be reluctant to chop off original loom plugs. Explore a loom ISO adapter kit for your car.
I would say go for one of the HU's Dave suggested (although I have no working knowledge of them) if you want to install a modern upgrade.
Cosmetically, with a replacement modern HU facia surround/fitment for an 'old Volvo' you shouldn't have any gaps!
Bob
