Basically, the problem I was having before was that spawning a Wither now requires air blocks on either side of the soul sand where its "tail" should be. So using the following key:

o = skull
s = soul sand
a = air
– = anything

I can say that previously, you could spawn a Wither like this:

o o o
s s s
– s –

And now it only works like this:

o o o
s s s
a s a