Yep I agree the beam in the car is the way, I had to trim the side of the diff to clear my beam.

I think the castings vary a lot as my rear case cleared the Mk mount by a few mm, and my mates cleared on the back and the side

Mine fitted all loose first then did it all up tight, positioned the diff to body mount, pinched with the two bolts you can still get to. Then dropped the diff back down and tightened the two that you can't get at ( front two ) then bolted the diff back up and bolt the diff mount up all loose, then tighten.

I have had my motorsport development diff mount counter sunk ( front bolts only ) as I only had 1 or 2mm clearance and didn't want the hassle of taking it out later if it did touch.