I ordered the Super Chevelle Coupe Kit from Ground Up and I FINALLY recieved all the parts.
I am not planning on putting in the headliner as I will have it done professionally. My question is should I put in the rear deck board and the rear deck insulation before the headliner is installed? Also, should I leave off the sail panels also and have that done when the headliner is done?

You can install the package tray before the headliner, but not the sail panels...

I would install the rear parcel shelf after the headliner and sails are installed.
Some models the bottom material on the sail panels tuck under the parcel shelf.