This set up will be the maximum width for sleeping. Top of the foam needs to be about an inch above the lower part of the plastic window trim. You will be using the window bays for head and feet. I am 5'10" and 1derGirl is about 5'6". 74.5" width on the inside of WINDOW to inside of WINDOW will be about right and that will be your widest point. Anywhere where there is plastic trim it will be covering a rib or structural piece that will result in a narrower width.
I was relegated to the rear most part of the bed and it was tight but we used that way for six plus years so it wasn't so bad. I could sleep more comfortably on the forward section of the bed but that was where she wanted to sleep. Also, keep in mind our van was an EB and the rear quarter windows and A/C duct created intrusions into the sleeping area. An RB will have the A/C duct work in the rear most driver corner so it will be less intrusive. (While I know the temptation is there to remove the rear A/C and Heater unit, I strongly advise NOT to do that. There is very little real savings in space and the loss of cooling/heating capacity from the rear HVAC is a huge loss).
The bed was 48" front to back and full width side to side. There was 18" clear height under the bed and 49" clear side to side. The ply was 3/4" (legs and platform) and the foam was 3" firm and 1" medium to create a pillow top feel. Worked great.
Window bays were insulated with friction fit panels. You can use Reflectix glued to Coroplast sheet and finish the inside face with material of your choice. These will provide a great insulation with minimal thickness.
I made it in three full width interlocking panels for ease of removal. I could have in or out in about 10 mins.