Alessandro, I have an oil cooler and remote filter on one of my Mustangs, the one I race. It is necessary there to keep the oil temp reasonable during a 45 minute race. My street Mustang that actually has more horsepower does not have one and oil temp is just right. I don't see the need unless you do a lot of high speed driving. If you are going to spend a lot of time on the autobahn, you may need it. I recommend an adapter that lets you adjust the angle that the hoses will point when installed on your engine.  They have a steel center that is made of steel and will not strip and come off of the side of the block. If you are in a cold environment, I would also have a temperature bypass so you get good oiling to the engine during warm-up. It is a lot of complexity if you don't need it. The oil cooler I run on the race car is a 25 row one that is low flow restriction and about half the size of the radiator.

The oil, like water in an engine has a temperature range it should run for maximum efficiency.  If you run the water temperature too low you will first have a bad running engine, and second you will wear it out much faster.  Hotter temperatures are more efficient as well, so the engine makes more power and uses less fuel.  The idea is to keep it from overheating, which with these older cars can be a challenge.

Oil wants to be in the 200-220 degree range.  At 250-260 it begins to undergo rapid thermal breakdown.  Synthetic oil can tolerate more heat without breakdown, but even it has its limits.  However, oil at temperatures below 200 doesn't flow or lubricate as well, so you are risking engine wear if you run it cooler. 

At the very least I would stick a gauge on it and see what the oil temp is now.  If its running 200-230 I would not add an engine oil cooler, because it will most likely just over cool the oil.  This may reduce the water temperature, but at what cost?