Had the same problem with our 2012 Lexi on an E-450.
Went to a metal fab. shop and bought some 3" square steel tubing. Had the fab shop cut it to length I wanted, and had them cut a notch in each end so I could access the bolts to tighten them down.
Basically I just made a big 3" spacer. The tubing was drilled at the same place as the floor bolt holes, and then I drilled holes on the top to bolt the seat frame too. Now the entire seat, (did both sides) are 3" higher.
A bit hard to explain, but pic's should show what I did.