When I added an external cooler to my 1973.5 this winter, I used mostly the 84-86 Carrera setup to guide me. Per PET:
Here, they call for Part numbers :
17 (6 ea.) 911 207 302 00
18 (1 ea.) 911 207 237 00
20 (3 ea.) 911 207 303 00
21 (3 ea.) 911 559 145 00
21 (3 ea.) 911 559 145 01
25 (1 ea.) 999 511 084 02
We fabricated our own pieces for items 17 and 21.
In the end, since my 1973.5 did not have the tabs that the Carrera has for the lines and the lines did not quite lay on the body like a Carrera, we had to get creative and use some rivnuts in different locations to get the lines to connect to the body. We also had to modify the oil line from the thermostat to the oil tank as it was about 1" short.