You need-HEAT- heat up the aluminum till its HOT with mapp gas, ((propane wont get hot enough)) then spray around it with something like kroil penetrating lube. I place a small peice of aluminum sheet on case surface so the pipe wrench cant contact the case surface. Grab the collar with pipe wrench, once you get it moving you can just heat it up and lube it again and they will come out.
you can vise grips but they wont grab the collar like a pipe wrench will, youll need to repalce the collars. smear a bit of grease on them when installing them and next time they wont be corroded in to the case and cylinders.