When I did mine, I did it when I had both the cylinder heads removed, so I had good access from above and below. I lost a good amount of skin on my knuckles trying to get the bolt to align on the second mount. I ended up using a pry bar to get it to align back up.
Its probably worth getting a helper to pry the engine back into alignment from on top, while your underneath taking care of the bolt.
You shouldn't need to remove the radiator, just unclip the shroud and remove the 4 10mm headed bolts holding the fan to the waterpump, and take the fan out of the way. You dont need to remove the shroud, just have it loose and the fan should slip past.

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Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i