Took some measurements today and also took a closer look around.
Looks like its about 3.75 inches down from where id want to cut the rocker. So like
@tRustyK5 said, 3" x whatever is definitely closer to stock size of the rocker but i think ill still go with 2" x whatever steel material. If 2x4 material is put flush with the outside of the bottom of the door, it will go in far enough to pass my rusted out flanges on the cab supports, but only by a bit and thats flush. Maybe 2x6 would be better as i could have it stick out a bit past the doors, and also i could cut more rusty flange metal off to mount them.. .When the inner rocker rusted out it definitely took that flange with it. That would save me from replacing 2 of the 4 cab supports, since they are solid enough except one that is cracked at the body mount and floating, but the other rear is pretty fubared aswell. Im not entirely sure how im going to replace the one in without removing the body mount, ill have to take a closer look at it. Maybe i could remove that body mount bolt and jack up the body enough to slide a new one in. I couldnt imagine id have to jack it up much, but welding it in that spot might be hard...
The bottom of the floors are good still, definitely need a good wire wheel , rust primer, paint and maybe some sort of spray. The sub frame support is still solid and cant poke any holes through it. Ill wire wheel that too and see if maybe it need some beefing up in the future. It definitely looks like some goopey inner rocker repairs was made at some point by a PO. Lots of goop and crap around the edges of where they were attached to the outters.
Also wondering if this support rod is removed or just tacked onto the new rocker/slider box, as the mounting surface will get cut ?
The rear inner and outer wells, and the quarters will get fixed next on the list so if this support rod is needed for them id like to keep it.
