3/4" cable should have been able to be forged out to about 1 1/4" - 1 3/8" wide by 1/8 to 3/16 thick. Did you twist it? you should take it up to red, put it in a vice, then twist it which ever way it naturally runs. do this 2x you'll double the diameter doing this, then flux and weld. draw your billet out of it and it'll be knife sized.
Jason