It was nice seeing your truck in person, you and other forum members too. Maybe someday after my foot heals there can be a get together down in these parts.
On your cab corners, and the rest of your cab. It looks really solid.
I would sand down the corners in question to bare metal to make absolutely sure there just as solid. You may also look real carefully on the inside.
If anything looks suspicious you can tap at it with a pic or something to see if it gives and then repair accordingly.
On the rockers. From what I've seen. Have your doors on where you want them that way the bottom gap will be correct. Keep the rockers in place with a few simple tacks and then open or remove the doors to finish up the rocker install.
I've looked at tons of threads and all have there bits and pieces of good information but Alan's Classic is the only one that comes to mind off the top of my head.
Alan's Classic link:
http://67-72chevytrucks.com/vboard/member.php?u=33050
This link will help pin point you to were rocker work starts:
http://67-72chevytrucks.com/vboard/s...=248460&page=2
So I hope this helps. Good luck. You do a great job.