On my 70 Mach 1, the drivers side floor pan needs to be replaced (from behing the drivers seat to the front where the floor pan starts to bend up). Someone suggested that with that much being replaced, that I should add a welded in bar to the door openings to keep the rest of the car from shifting or moving out of shape. I have one door removed from the car for restoration. I am having no luck getting a true/false answer. Anyone heard anything? Thanks.  

I've never heard or seen that in any of the books I've read.  But, as a side note, it is true if you're doing a convertible.  But it's probably not a bad idea if it gives you peace of mind.  

No support would be needed even if you where doing a convertible. The outer rocker and the top structure is plenty strong enough to hold things together and on a drop top there is also an inner rocker. If the repairs become more extensive like say replacing the torque boxes then i would consider some sort of support, we have a jig we put the bodies on that can support the car in several locations if needed.The only time we add supports to the door openings is when the vehicle in placed on the rotisserie.