Look at my build thread (link below) post 54. first post is how I built the inner cab corner panel (they were not reproduced then) next one in the interior closure piece. Some guys do not put the lower inner panel back, but I will be using my truck off road and wanted the extra support and to keep the crud off the exterior metal.
http://67-72chevytrucks.com/vboard/s...7&postcount=54
Note that I spot welded the inside filler to the exterior skin because I had not started the body work there yet.. you will not want to do that, a good weld to the floor and good sealant will do.