I have a Stable Energies Harness Bar in my '73 Targa (
http://www.stableenergies.com/prodinfo.asp?number=GB1). It is easy to fit and works well with my Schroth ASM 4 point belts. It takes me about 5 minutes to install or remove it. It also allows me to use my stock 3 point belts on the street.
Mine is no longer in their catalog, but this one is listed:
https://www.stableenergies.com/harness-systems/4886-brey-krause-harness-guide-bar-porsche-911-targa-74-93
Bear in mind you need to be careful since harnesses are not compatible with many of the Porsche seats.
Here is a shot of mine:
This install is not legal for DE due to the high back seats which interefere with the routing of the shoulder straps.