I do the floor pans first. You will discover the condition of the front cab mounts under the floor this way. You need to ensure that you have a sound foundation of cab to frame engagement before you move forward to the other parts. If you don't have this, you will experience some weird shifting in the structure of the cab as you start cutting hinge pillars and rocker panels.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission