Room Accoustic and Wall Panel?

I am not sure I understand your plan, but a simple way to reduce the panels protruding into the space is to remove the wall covering (sheet rock ?) and fill the space between the studs with the acoustic fiberglass.

The studs are already 3.5 inches of your 6.3 inch requirement. Perhaps extending the studs with another 2X4 and then covering the entire area with acoustically transparent cloth.

The area where the studs are would be a staple point for the cloth, and then decorative wood strips would cover the attachment area.

There is also the option of a rail system to capture the cloth, but this is much more expensive. I will provide the web link below so you can take a look.

http://www.owenscorning.com/around/sound/acoustic.html

http://www.soundcontrol.com/whisper.htm

Albert,
My plan is to leave the the existing wall on, and to build a new frame using 1"x4" pine pack with R-19 fiberglass, polyester, and cover with Guilford fabric. The reason I'm doing it this way is in case if I want to remove it in the future all I have to do is remove 4 screws and the panel is off. What do you think?

Nothing wrong with that at all. I suggested the other method because I thought you were determined to have 6.5 inches and that would begin to invade your space.

You should consider Owens Corning or Johns Manville acoustic fiberglass instead of R19. It is packed denser and will not migrate into your "breathing" space through the Guiford fabric.

I believe my links will take you to where you may investigate this option. Other good news is that 1" of the acoustic material is nearly as effective as 4" of common insulation. The acoustic is available in 1," 2," 4," and 6".

A recording studio that I recently photographed had all 6" plus a strange box and Helmholtz trap in the ceiling. It was absolutely amazing what it did for the piano in the cavernous recording space.