....I have been thinking of putting a complete nissan s13 subframe with diff and everything in it. Does anyone know if it have been done before?
It's been done in a 510 station wagon, which is very similar underneath. If you have the guts, it's possible.
And is it room enough under there?
You will have to narrow the S13 suspension crossmember at least 180-200mm to fit it without flares. Obviously, the axles have to be narrowed the same amount, but if you can measure and weld it's definitely possible.
There's some pics of the install in the 510 wagon here:
But this isn't the sort of thing someone can just tell you how to do on the internet. You need to get your hands dirty.