Don't loose heart mate, it doesn't take much to make the standard beam fit. Do you have any competent friends who are engineers or handy with a welder even? But it gets more difficult if it originally had a WRC style beam like the one posted above by Coley.

Offer the beam up and bolt in place, the diff mount will be easier to modify. I made one to fit another car, and its simply a case of getting spacer correct so the mount sits flush on the boot floor. You won't need to worry about the angle of the driveshafts because the diff will already be in its right location when bolted to the beam. But you need to ensure the diff mount is secure.