If I cut a 10.5 to 8.5 will I have to widen the GB hole?
Has anyone done this?

All help and advice appreciated.

The issue will be the gas port location as the 10.5 is likely carbine length and a 8.5 will need to be pistol gas length.   All is mute if you weld over the old hole and drill a new one.

I did something like this, I drilled the new hole and the old hole was covered with a front site. Welding it closed would be more professional looking

If your existing gas port is pistol length, you may not need to change the port size because the percentage of change in dwell time won't be all that different.

But if it's carbine length gas, you probably won't be able to open it enough to get good cycling because of extremely short dwell time. And getting a gas block to fit the barrel without extending past the thread shoulder is going to be another problem.

I can't even imagine it functioning if the gas port is in the carbine spot. A lot of people won't even run a 10 or 10.5 do to reliability issues from the dwell time.

The gas port hole would be at the end of the barrel and would not fit any gas block without half of it hanging past the end of the barrel.  
Welding the gas port and making a new hole closer to the receiver would be the only option.  Either pistol length or a custom gas tube would be needed.