I like the fact that you are using Ghoultree and running Spider Spawning , which most people with this type of build forget is a solid back-up plan but this deck desperately needs Varolz... between him and Lotleth Troll , they are the staples of this build. I will give you the +1 just for the Spawning.

One solution for the Ghoultree chump block, is to add red to the deck and use Kessig Wolf Run another solution would be add 1 Rogue's Passage . But Rancor is probably the best Idea. Especially with all those 1 drop creatures you have.

Solid zombies with a number of them staying around long enough to block until you can cast Ghoultree but it's risky waiting for natural mana pulls to get you to 8 to cast him with only 20 lands in the deck (only my opinion there). A few decks i've played with this design use things like Mulch , Grisly Salvage to get land and place cards in your graveyard. Also cards like and Caravan Vigil and Farseek help gain land. More land equals quicker Ghoultree and with creatures with scavenge in the graveyard that can only serve to pump others!!

One other card i'd mention Jarad, Golgari Lich Lord , think about it.