I am replacing my floor pans etc with the complete floor and platform kit from Wild Horse. I think leaving the door pilar and just cutting the bottom is easy, if I replace the door pilar then it would make send to have all new metal behind it. My fear is the how good are my measurements etc to put the pilar back exactly where it needs to go.
Hard to tell exactly from pics, but I suspect your bracing will be in the way if you are replacing the whole door post. And if just doing the bottom of the door post, I would also brace from floor up to dash, or floor up behind dash to cowl. Whatever it takes so the dash, cowl doesn't sag when you cut out the bottom of the post. Just hard to know what will happen to all the rusty parts when the door post bottom is gone.
I had similar rust and I just did almost the whole body. Only metal remaining original on my body is upper outer quarters of the tub, top of tub, the passenger taillight housing and the doors (which I blasted, rust inhibited and re-skinned). The rest is all new body parts, done in sections. Get one part installed, aligned, then move on is how I did it.
It is a lot of work, but I will say that knowing the body is 100% feels good in the end. I reworked the whole body, had everything lined up, only then removing the body to do the frame.