The engine arms of the 4.5 will bolt on the 5.6. As such the engine with sit exactly in the same way as the 4.5. No mods needed.

For the transmission, it's a bit trickier: The pitch of the two bellhousings is different. As mentioned by Frank, the two top bolts are 8mm more inwards the V. With a bit of luck your 4.5 bellhousing is one of those that was designed to accommodate both bolt locations. But you will need to enlarge the holes in the intermediate flange. The other issue will be with the oil pan, as the trans is supposed to bolt on it on the 560. Here, you cold try to fit the 4.5 oil pan and the two brackets that link the intermediate flange to the block from the 4.5. All the oil pan fixing holes are not in the same locations, so you may have to do a bit of drilling or taping.

You could use the 560 trans.

The 4.5 intake will bolt on the 560 heads. However the diameter of the runners is slightly smaller (39mm instead of 41mm). You will also need to grind some material out of the plenum, in the front and in the rear right corner, You will also need to grind some material out of the block where the rear right angle of the plenum sits.

The W126 exhaust manifolds won't clear the w108 engine bay. You need to use some r107 560SL exhaust manifolds.