Head stud drilled out.. what now!!

[Aussie16/70]

Hi Brains trust,
I've got 1 dodgy head stud that needed replacing and wouldn't come out (yes I did all the heat and soak etc)
I have drilled out the stud on my CNC with an Ø8mm drill which is just under thread minor diameter, and tried to run a tap down to remove the remaining stud, but there is next to no thread left.
So my options as I see them are
1. Drill out the hole to Ø11 and use a modern 3/8UNF Helicoil then make a high tensile stepped stud(3/8UNF to Ø11 shank to 3/8" stud to 3/8-20BSF. 
2. open hole out to 7/16BSF (again Ø11.1 clearance hole with stepped stud)
3. file a flat on the bottom of the hole and use a nut.
4. a good idea you've used that I haven't thought of yet.
I'd appreciate any experience here.
thanks
Tony