I'm waiting for paint to dry, so I did some rough calculations for you...
Assuming the actual dimensions of your back chamber is 39.5" long and 3.75" wide, and assuming the operating depth is 15", then you'll need just under 10 gallons of space in your sump, plus the volume your tank will drain (if it drains an inch that's a little over 3 gallons) and pipes/skimmer (shouldn't be more than 2-3 gallons), etc. So you need a total of about 16 gallons of empty space in the sump for
full redundancy. If your sump is say 22" wide and you need say 9" depth in the sump, then if your sump is 24" long it will have 21 gallons in operating volume, add to that your 16 gallons, and if it's 16" deep (15" to be safe) you're probably going to overflow. If you made the sump 30 x 18 x 16" your operating volume would be 21 gallons, and total volume would be 35 gallons (to 15" depth). 21 + 16 = 37 gallons. That won't work either (for full redundancy). Play around with the numbers and you'll figure something out. Don't forget to consider the frag tank too. That'll throw a wrench in it.
36 x 18 x 16" total volume (to 15" height) is 41 gallons. Running volume at 9" depth is 24 gallons. Add 16 gallons to 24 is 40 gallons. Too close for comfort, and certainly won't work with your frag tank.
36 x 22 x 16" total volume (to 15" height) is 50 gallons. Running volume at 9" depth is 30 gallons. Add 16 gallons to 30 is 46 gallons. Hey, that might work...