I believe they are a M22x1.5 standard thread but they have an interesting sealing method, like an AN has its flare seal but these are more rounded. That's why i reused the Benz fittings.
Might be able to use a regular metric fitting though.

 

This is the one I used. Had to fab up a couple of mounts to existing holes in front.

http://www.summitracing.com/parts/fld-db-30216

 

I'm using a Setrab 51-06663, which is about 22" by 6.5" and has the ports on the side.

 

The Fluidyne DB-30618 may work as well, it's 21x3x5.875", $379 from Summit Racing...

 

Are you running an evans waterpump? I'd be concerned about covering up the radiator too much on a motor that is known to have cooling issues to 7.

 

I do have an Evans water pump, with dual fans and the RH/RD radiator.

 

I have an oil cooler from a BMW M70 V12 engine sitting on the shelf that I intended to use in my stillborn 944 Spec car. A Google search will show pics, it is designed to fit under the radiator and even has metric fittings.

I know that a used oil cooler is a gamble, but several BMW vendors like VAC Motorsports and Turner sell new BMW coolers for under $400 if that is not too expensive.

On my 951 I mounted a second factory cooler in series on the other side of the radiator and it works great on 100+ degree days on track.

 

Decided to go with the Fluidyne core, it'll fit well and I shouldn't have to change the hose ends or even the hoses to get it to work.