Thank you for your reply.
For the first question, I tried to follow this guide, but there is no gauges in the picture generated (or I did something wrong, the picture was weird).
The answear for the second question, GTN in 2d pop up window works flawless with the FJS 727 Autopilot.
I tried to replace the FMC obj files with the GTN750 3d obj in planemaker and it kind of worked(In planemaker the GTN750 3d obj showed up instead of the fmc), but when I loaded in X-Plane, there was just an empty space with the FMC screen and invisible buttons. I tried to this because I didnt now how to calculate the coordinates to put in another place.
Finally, here is a video taken this month of a real 727 with GTN750 flying in Brazil.