One thing I did, was to use a "Card Board Template" of the door opening, before I cut anything....the credit for this actually goes to Vic Cook and Scott Duckworth, they had done this on Vic's project truck. Scott had a pristine cab that was almost rust free, he took a couple pieces of tri-wall card board and glued them together, then fit it to the inside of the cab against the door opening, traced around the pinch weld and that's all that's needed.
before I started cutting on my floor/rockers/cab corners, etc... I braced everything side to side, front to back, up and down, to ensure nothing moved out of place while all the panels were being removed. I was told by a friend that you basically, start on the inside and work your way outwards with regards to sheet metal replacement. my rear cab supports were so badly rusted, that there wasn't anything to weld the replacement cab mounts to... I ended up buying the entire rear cab support assembly. In hind sight, I'd probably had been better off replacing the entire floor pan, I would have got all the cab supports/braces and inner rockers in one solid piece, sure would have saved me a ton of grinding on all of the those welds too.
If you have access to a portable soda blaster, the one thing I found that was neat about using baking soda, was that as you blasted away the paint, the "Burn marks" (spot welds) show up clear as day, making it so much easier to find the spot welds then having to sand/strip the paint away to find them...make sure you read everything about using baking soda before you start doing any paint work though, because if you don't get rid of the residue from the baking soda, it will cause problems with your paint later! They make products that will remove the residue from the soda......
And probably the single most important piece of advice I can give, make sure you have your doors on before starting the rocker panels, or you'll regret it later, trust me.....this is the only way to ensure you have the correct gaps.
Once you have "mocked" up your rockers, use the door template to gauge how your panels line up.....this sure helped me tons.....
Ben