If you're adding a level kit, why would you need to change the rear block? Maybe they meant that the level kit is level with the F350 rear block setup, and that a F250 block might be nose high? If you want to level without going F350 high you can use a 1" spacer instead of 2".
I think a decent level setup for a 2005+ F350 is:
2-2.5" front spring spacer
caster bushings
Longer front shocks
Stock sized rear shocks
Adjustable track bar
The F250 is the same but with taller F350 rear blocks added, or a shorter 1"-1.5" front spring spacer and stock F250 blocks.
I thought about using a level kit, and switching to F350 rear blocks so the rear is level or taller, and adding a front adjustable track bar to make sure the axle is still centered. After totaling that all up. It wasn't so cheap anymore. I'm going to stay stock for now, and then just spring for a complete 4" system next year.
And the 4" kit uses front springs and a slightly larger block in the rear. So your stock rear leafs are in the stock position, so airbags still work. And if needed, when I add a winch, BDS has a 1" spring spacer for up front to lift the nose back up after the weight of a bumper & winch pulls it down.