I have a similiar issue but mine is on a bank...
My plan is to use a winch on a snatch block with a y cable and chain setup..
Chain on the base where the tree is resting on the stump and a chain about where you have the Y..
A steel cable connecting the 2 chains with a pull loop set 2/3 towards the Y..
the snatch block to the winch will be at a 45 degree angle..
This will make the tree pivot on the stump slightly as it pulls it foward to drop it off....
Once I get the tree off the stump, I'll use another chain going to the tractor to pull it up the bank so it can begin the trees final journey to the ash bucket....
In your case some creative thinking with a winch, some chains and a snatch block may really help out...
thoughts?
J