Radiator conversion for 1980 240D

[SWjames]

So, I bought the wrong radiator for my 240D and I'm curious about what it will take to just convert to the radiator I now have rather than deal with the hassle of returning it. What I bought by mistake was the radiator for the 1982-1985 turbos in the W123 and 126 chassis. I understand that I need to add the overflow tank because that's where the filler cap is, but I'm not sure I know where to put it. It seems like it would have to be the highest point in the system, and I'm not sure how to do the initial fill after the new radiator is installed. Also, can I get away with continuing to use the regular green coolant or is there a compelling reason why I should spend the $50/gallon for the special MB coolant?