I have a special cutoff wheel for flush cutting, and I still lose 3/16" of metal so that's a concern with using the existing mounts - you can use some wide strap (1/8" or 1/4" X4") between the frame and the mount to make up for the lost metal. Also, make sure the the V8 mounts off the frame mate up with the engine mounts on the 200tdi. Finally, make sure the hoses fit with your radiator to engine spacing. Hoses can be shortened, but making them longer is hard. Also watch the exhaust down pipe clearance and it is nice if the shifter still fits the tunnel in your 90. Depending on where the transfer case ends up, you may need new driveshafts.
Personally, I would cut off the mounts - mount up the engine/transmission/transfer case to the transmission mount and see how the engine to radiator spacing is. The less you have to modify the better. On my 300tdi the engine mounts were all I had to move on my 90. On my 200tdi I didn't have to move anything, but it was to my lightweight. Have fun