Some time ago I found another cool website that shows the m52 swap process: E28 Goodies. In a blog post on that website they mentioned an E36 AC radiator, that particular model fits the standard gap in the E28 front better than a 524td radiator. Spot the difference:

As you can see, the expansion tank is integrated in the side of the radiator. This means two good things: I don’t need the external expansion tank any more and even better, I can use the original hose to connect the coolant return tube to the expansion tank!