FWIW when we were looking, Bryan said they were flexible with the thickness of the insulated floor construction. At that time, IIRC, the standard was 2x4 spacing, 1 1/2" insulation in the cavities and a new 3/4 ply bottom. Call it a 2 1/4" addition. I was interested in an insulated floor for cold weather and added floor strength but I think just adding a second 3/4" ply layer would help for both.
If you could put 2,000 lbs or so in the truck box now you'd have a better idea on the spring deflection.
