1 as previously stated, the chassis has flex along the bottom rail. This causes the doors to foul the door shuts and can be rectified by adding 40x20 box section to the bottom of the main railes whilst under tension. This is where I went wrong on my project. Car needs to be fully assembled and at running weight. Hold the pre formed box section against the lower edge of the chassis with a trolley jack and, lifting from the centre of the door opening, with the doors closed go up slowly until the chassis starts to lift. Be very gentle and go up slowly until the door is a 50p's thickness higher than the rear door shut edge. You can now start stitch welding the box section onto the lower rail VERY carefully.
When it's lowered back onto its wheels this gap should diminish or vanish as gravity and the existing curve of the chassis (and they are all curved) It may still be up a bit but this will go when you get in the seat. Once happy with the fitting, finish welding. When both box sections are stitched on get it up on ramps or axle stands under the axle / front lower arms to seam the inside edge. Make sure not to damage the bodywork or any pipes / wires in the process.
The second reason is that the mild steel door tubes have rusted out. This happened to my drivers door whilst we were on tour and I bodged a fix, that bodge is still in place but a new hinge will be forthcoming once I cut the assembly from a donor door to see how it works and replicate it in stainless or ally. That won't be for a while yet as i'm half way through restoring an S4 Phaeton.
Yes. DO NOT use the same method as I did, it isn't accurate enough however take a look at my flickr gallery here where you can see the work I have done on one of mine. Once you've had a look at the photo's we can discuss stuff.