Both, the 4.11 diff and the 3.64 diff were made in long neck and short neck configurations. The 1968 to mid-69 2002 was equipped by the factory with a long neck 3.64 diff. The 1600-2 from 1966 to mid-69 was equipped by the factory with a 4.11 long neck diff. At about mid-69 BMW switched to the short neck diff configuration, not because the short neck is better, but rather because it was cheaper to produce. The long neck is a better design as it holds more oil and has a large hex nut deep in the neck for the crush collar. The flange for the driveshaft is held in place with another large hex nut. On the short neck design the crush collar and flange share the same hex nut. And, once the hex nut is loosened, the crush collar must be replaced. To replace it the diff must be disassembled. This isn't the case with the long neck design where you can replace the oil seal without worrying about the crush collar.
In the US the short neck is most readily available in 3.64 ratio. It's difficult to source in the 4.11 ratio because BMW ceased importing the 1600-2 into the US in 71. where the short neck 4.11 it was only available for about 2 1/2 years. The long neck in 3.64 was only used in the 2002 for 1 1/2 years from 1968 to mid-69. So, it's even more difficult to source. But, the long neck in 4.11 is easier to source as it was used by BMW on some of the NK cars and the 1600-2 from 64 to mid-69. The short neck 3.64 is easiest to source.
If your 1600 is equipped with 3.64 diff from a 2002 and you are using a speedometer geared for the 4.11 diff, it will indicate a speed 12% slower than the speed you are traveling at. On the other hand, if you are running a 4.11 diff and you use a speedometer calibrated for a 3.64 diff, it will indicate a speed 12% faster than the speed you are actually traveling at.
12% of 45mph = 5.4mph.
12% 0f 75mph = 9mph
This is a significant error. If your short neck diff is a 3.64, I suggest you use the speedo from your 69 cluster because it appears to be calibrated for the 3.64 diff, judging by the red maximum speed mark on it for for 3rd gear, which is at just over 80mph. Notice the 3rd gear maximum speed mark on the 1600-2 speedo is placed at under 75mph.
You can get the speedometer recalibrated by a speedometer shop. But, it's not cost effective as the price for a used speedo in the correct ratio is much cheaper.
If you are still running a 1600cc motor with a 3.64 diff, you don't need a 5 speed trans with overdrive in 5th. because you already got your 12% overdrive through the diff. It's actually a good combination for the 1600.