Although I absolutely love the concept, I can see a few challenges. In no way am I intent on discouraging you.
The width of a Suby motor may be too wide for the narrow 7 frame.
A 7 is traditionally rear wheel drive. What gearbox do you have in mind?
The engine of a 7 is set behind the front axle line, which is way up front. If you are thinking of going all wheel drive, the nose of a 7 doesn't have the real estate for an engine ahead of the axle.
My thoughts off the top would be a built EJ25 or the 6 cylinder options suggested above, mated to a Nissan or Toyota box, diff is open to options as you need a custom driveshaft anyway, but matched to rear suspension if its not custom. Rear suspension and shafts you'll need to research. Old school is a solid rear diff. I've seen Mazda MX5/Miata rear subframe used successfully.