You can fit door handles for Datsun 240Z 510 (front) to 510's rear doors (4 Door and Wagon model) by making some modifications shown here. 

1. Cut and remove the pin out of the old rear door handles.

2. Use 5/32 Drill bit (The same ID as the hole on the other side of the pin) and drill a hole on a new handle.

3. Make sure that you are drilling straight in.

4. Now, press in the pin that you removed from old handle