The key aspects of the cooling system include the radiator, radiator hoses, heater hoses and overflow tank.

We will use the stock 300D radiator in the stock position, so the upper and lower radiator hoses will work with no modification. The heater hose connections to the chassis are in different positions so we will need to create ways to connect these together.

The key aspect of the cooling system is the radiator, and it turns out that this is extremely easy to adapt to the 380SL.

Radiator

The first thing we will do is cut out the lower radiator mounting from our donor car. We'll then fabricate some small brackets to hook this to the 107s chassis.

Here's the finished product.

The trick to getting the right is to fabricate the angle brackets for the car first and install them. Then fabricate the angle brackets that will attach the lower support to the car brackets. Clamp these in position and then mark the position of the lower support relative to the center of the car.  Then drill and pop rivet the brackets to the support member. Now hold the radiator in position and clamp the two angle brackets together with the radiator in its proper position. Then drill the two angle brackets and bolt them in final position. 

 Here's a top view of the finished installation.

Finally, carefully cut slots into the front support and fabricate a clip to support the top of the radiator just like stock. I found that I needed 1/8" of clearance to keep the radiator from hitting a mounting bolt for the A/C, so I pop riveted a strip of 1/8"x1" aluminum strip behind the front support from one clip to the other.

Here's the finished installation.

Overflow Tank

For an overflow tank I used a stock Mercedes tank. It was tight but I mounted using the existing bracket for one end and fabricated a bracket for the other end. For the hose to the lower radiator connection, I used the stock hose but had to shorten it by splicing in a piece of tube I found at the junk yard to give me the proper length and routing. 

Heater Hoses

There are two heater hoses that need to be connected. One is at the back of the engine and wraps around the oil filter and the other one runs from the aux water pump on the firewall to the front of the engine by the thermostat housing.

Here i used a length of pipe I scavenged from the 300D donor, so I didn't have to run soft hose the entire length.

For the heater hose by the oil filter I used sections of the stock hoses and just joined them with a 90 I salvaged from the 300D.