To answer your first question, there are 3 versions of sensor brackets.
The early cars had a 2 hole version--speed & reference sensors. Should be fine for your early car.
Late cars had a 3 hole---speed, reference & TDC sensor
You can see from the picture the sensors are different. The left "adjustment" hole is different.
The final version had only one sensor----for the 968s and 944S
The sleeve you refer to was installed to sheild the sensors from the electrical interference caused by the high torque starters. This is also a reason
not to cut out the bell housing sensor/bracket area. The bell housing helps sheild the sensors from the interference.
GL
John