Doesn't sound very hopeful, I am afraid!!!
That is alot of damage.
You could still try the grafting though. Grafting is usually used to join two plants together so that they grow together as one. The soft tissue beneath the outer bark grows together, much like our skin does when we have a cut.
You need to bind your injury with raffia or plastic tape, or maybe even a piece of plastic, as yours is quite large, completely around the entire wound. Make sure that your bark is a little moist before doing this as nothing will happen if it is too dry. Once you have bound it, then cover the whole area with graft wax from your garden centre to seal in the moisture and prevent infection.
This takes quite a bit of time, so be patient, and there are no guarantees!
However, remember, that even if the tree above the wound dies, a new tree is likely to shoot up so watch for those to come up as well.
I hope all goes well!
VAL (Grandpa's Rose)