CS 707 is a favourite of mine as well. Used to use it in FSX-SE with triple Simufly INS and later with the ISG FMS.

Check out the General Discussion thread. P3DV4 upgrades list on the first post appears to be actively updated and has changed a few times in the last few years, but not long ago the 707 appeared as "in progress" for P3DV4, and a few days ago this changed to "standby".

Of course this could change again to something less promising but for 707 enthusiasts this is interesting.  :)