what you could mount at the rear is a complete rear axle from a P4. Problem is the suspension setup. Tunnel isnt a major problem , although you do need to chop off lots of the floor.
For starters, you'll need to cut off the spare wheel housing at the rear, remove the fuel tank and put a homologated one inside the car.
Then moving forward, the engine is a LOT bigger than a standard boxer engine. I've got a 2.5ltr V6 engine at home and its MASSIVE !
To put that one in a Sprint, you will need to chop off everything inside the engine bay ! the other thing that will need extensive modifications, is the gearbox of the V6 engine.
So , in other words, for that setup, you probably need to find an engine and gearbox of a rwd car. Think Alfa 75 V6
the gearbox of the 75 will be at the back along with inboard diskbrakes and handbrake at the back as it should be. Mounts for the gearbox and shaft would be much easier to make, as the gearbox wont be where the drivers seat is. Gear lever wont need much to work either as the rods will come from a rwd car and you can swap them right off !
However, although I must admitt I did think about doing this as well on my red Sprint, I gave up on the idea as the car would be nose heavy. I found it more convinient to put the 16V engine in and supercharge it to get as much power as I can from it !
thats how I got my username - Supercharged16V