456 GT Steering Rack Bushings

My Euro 1994 456 GT has been diagnosed with deteriorating steering bushings (see picture below), and I'm having trouble sourcing replacements. Would someone be able to confirm that the part number for these should be 135139?

Also, while looking for info on this issue in other FChat threads, several 308 owners seem to have replaced the rubber bushings with PVC or delrin ones, namely:
https://www.ferrarichat.com/forum/threads/308-steering-rack-bushing-replacement.260688/

Has anyone here done a similar change on a 456/550/575? If so, two questions:
1) Where did you source the PVC or delrin bushings?
2) Were there any negative impacts on the steering feel with the harder compound bushings?

Thanks in advance for your help!