Welcome to the forums from Kansas City, KS.
First off, start with bracing the doors with 1" square tubing across the door opening at 6" up from the door sill. Second, with a pointed body hammer or like object, tap around the perimeter of the rusty areas, to determine where the bad stuff ends and the good stuff begins. Locate the spot welds around the perimeter of the outer rocker panels, center punch them and drill them out with a spot-weld cutter. After the outer rockers are off, you will have a good look at the inner rocker panel and a better idea of what needs to be replaced.
At this time, if your budget allows, have the entire cab media blasted. By doing so, you will remove all the rust and start with good material. I would then cut out the damaged areas. From what I see in the photos, you will need front floorboards, kick panels and outer rocker panels...you may also need inner rocker panels.
There many sources for these panels, however "let the buyer beware" because all patch panels are not created equal. Here is a list of suppliers of aftermarket parts for the '60-'66 Chevy trucks:
www.bowtiebits.com
www.lmctruck.com
www.classicparts.com
www.oldchevytrucks.com
www.ecklerstrucks.com
www.gspp.com
www.truckandcarshop.com
www.harmons.com
www.oldmusclecars.com
www.classicchevroletparts.com
www.classicindustries.com
www.bobschevytrucks.com
www.brotherstrucks.com
www.mar-k.com
www.wheelvintiques.com
www.totallystainless.com
www.earlyclassic.com
www.horkeyswoodandparts.com
www.raybuck.com
www.americanclassic.com
www.yogisinc.com
www.rbsauto.com
www.obschevy.com
www.cheyennepickup.com
www.chevypickupparts.com
I just bought floorboards, kick panels and outer rockers from
www.lmctruck.com and I was happy with the overall quality, fit and finish. Be prepared to do some old-fashioned fabricating, body-dollying and wire-welding, in order to make things work properly.
In closing, there are a number of great build threads, explaining this process in a very informative way, so I encourage you to do a search of the site for this information.
Good luck with your project and keep us posted of your progress with lots of photos.
PS-I would start with the panel replacement Play-by-Play in Alan's Classic signature. Sorry Alan, your thread slipped my mind.