Double layer of 5/8" drywall outside, 2x4 studs with R-15 fiberglass insulation, and another double layer drywall inside.
If 8 ft tall walls, on the inside, the 2 ceiling drywall layers will reduce the height of the inside-wall drywall by 1.25 inches. An 8 ft drywall sheet will have to be cut down by 1.25 inches
In my experience, very thin cuts of drywall are difficult. The best idea i have for this is to clamp the 1.25 inch strip it between two 2x4 studs to attempt a clean break, then a drywall rasp to smooth it out. It's untested and I still don't think it will work very well.
It's much easier to install a 6-ft sheet followed by a 2'1.25" sheet. Or a 2'3" sheet. Or any custom size necessary.
Two of the walls have only one-side access so instead of hanging drywall on erect walls, i have to raise 4-ft-long sections of the single-side-completed wall by myself.
I can't make the walls 8'3" and put full 8x4ft drywall sheets because there will no contact at the top and bottom.
It seems the ideal wall height may be 8'1.25".
In that scenario, the outside drywall may have adequate contact on top and bottom. From my first build, i discovered that an air-tight seal is very important to block sound. What do you think?
I've been able to buy 9, 10 and 12 ft 5/8 drywall in the past. Don't limit your thinking to just 8 ft. also they make 54 inch wide drywall for horizontal installation. Check with your local drywall suppliers (not the big boxes) for availability. When you are forced to cut off a small amount, score the drywall on both sides first, then you can usually snap off an inch pretty easily.
Although i value every inch of high ceilings, it's probably slightly better to do 8' 0" height; fewer customizations, better fit with less handling.
This is assuming a full 1.5 inches of contact on the top and bottom of the outside drywall layer is significantly better than 0.75 inches of contact (if walls were 8' 1.25") when perfect placement and fastening isn't guaranteed.
9-ft drywall and lumber is a good idea. it would have saved me so much trouble on my first 8' 3" tall build. My trailer is only 8-ft so in my case i'm limited.