Rocker-how far should I cut?

[1930artdeco]

Replacing the top of the rocker due to everyone’s favorite chemical reaction. Question:how far can/should I cut out of the top of the rocker before I have to brace the door frame? Here is my plan:

1) replace the lip of the inner rocker panel.
2) cut the top off of the rocker completely. And then place half of the top back on-weld and then paint the inside. I have a gun with a long flexible tube for this. Put the other half of the top on and paint the rest.

Will this work or do I need brace the door? I can put a jack under the rocker frame to stabilize it if need be.

Thanks,

Mike