how to lay a foundation instantly

[vespertine 0]

ok  so i see how to clear a big piece of land instantly using the Z command, but, how do i then cover that freshly flattened land with concrete blocks just as fast....i tried using the Z command again while having the blocks equipped but it didn't work. i dont want to have to lay 900 million blocks one by one to make my foundation for my building. anyone know what key command or how to get that done?

[Telric 237]

Select with z, hold w/e block you want to fill and hit L. That will fill the selection with the currently held block.

[Ti2xGr 74]

You can't do it without being in the world editor I believe. In there, I think it's "L" to replace the layer with what's in your hand.