The Porterfield PMB-AP45R4-S pads in the catalog are listed for the fronts only, not rears:
1983 Porsche 911 SC Coupe - Brake Performance Upgrades - Page 2
According to the Porterfield website the AP45 part number (pelican # PMB-AP45R4-S) works for Front for PORSCHE: 78-89 911 and lists nothing for rear application. The part number AP31 on Porterfield (pelican # PMB-AP31R4-S
HERE) lists for REARS FOR PORSCHE: 78-89 911. Hope this information helps.
What brake fluid do you run? I'd highly recommend trying out the Ate Typ200 fluid (706232-GOLD
HERE). I run this fluid on the street and track and have never boiled it. It's cheap and works as anticipated. That fluid with SS lines will do wonders.
Let me know if you have any questions.