what is the best tool to grind down some of the webbing on the block so that my turbo fits. also how much of the webbing can i actually grind before i've gone too far, and also will i be alright if the turbo hits the block a little??

 

grind it with whatever you have, right angle grinder, dremel, ect.
use common sense while gringing on the block.
i wouldn't want my turbo touching my block, but i gues it depends on what your manifold is made out of and the expansion rate of that material.
just my 2 cents