You are on the right track. Given the lack of connectoin between your hinge pillar and anything below the dash you will need to get it located correctly before getting it secured to the rocker panel and front cab mount.
Replacement panels are not a very good fit typically and should not be used to locate the critical parts (such as the hing pillar). Rather locate the pillar and then massage the panels to fit.
Yes, I am finding that once the bracing is in place I am now spending a long time measuring, checking, remeasuring and so on. With nothing connecting the door pillar to anything, I am studying the FAM to try to understand basic things like how far forward the new rocker panel should protrude behind the hinge pillar. If anyone has that measurement it would be very helpful. It looks like the front end has to line up with holes in the inner cowl panels which themselves line up with the front edge of the outer cowl panel.
I am planning on simply mocking up 4-5 panels using self tapping screws to see how they fit. Very conscious that I can measure/move multiple times but ONLY until it is welded so that will be some way off.
