If you have an 04, you should have leaf springs in the front. If you have an 05-up coil front end, adding a spacer in the front, and a block in the back is fine, although stacking blocks isn't recommended due to axle wrap. Since you have an f250, just get some f-350 rear blocks, they are 2" taller than the 250 blocks, and will ad some height without stacking!
My lift in my 05 started life as a 10", and I added a 2" spacer (below the coil spring) for the front, and a 2:" spacer in the rear, bringing it to an even 12". Works great, but keep in mind it will offset the front end a bit to one side, so an adjustable track bar will be needed.
Also, you will likely need longer brake lines and shocks, as 8" is pretty much the maximum the stock lines will work for. You will need to re-shim the carrier bearing, adjust the track bar to center the differential, and in the case of leaf sprung front ends, you should consider crossover steering over 8" in order to keep everything at the right angles. The bigger you go, the more you will spend. Be prepared! 8-9" is pretty much the max without some significant mods!
BTW-40's won't fit with a 6,5" lift. 37s are pretty much maxed out, 38.5's clear with an 8.5" lift, but 40s will rub on a 99-04. I have 12" and can still rub my bed skin under maximum articulation!