A lot of good ideas filtering in!
Some thoughts..
1.0 You might want to do a layout of each door to find the "smoking gun"
Take same measurements such as along the door belt line, door hinge fixing points on each door etc... to establish what is not the same to eliminate the cab as the issue.
2.0 Before I performed door fitment and then welded the cowl panels I placed the cab back onto the frame in the constrained condition at the 4 fixing points.
That is a robust cart you have but is the cab sitting properly flat on the cart?
3.0 That door hinge pillar does sit forward at a slight angle to the bottom door sill. Might want to check that angle with a protractor on the drivers side then compare to the passenger side. Might have moved when original cowl panel was removed.
