Thanks again Chris, makes a lot of sense. Your approach is pretty much what I have been planning on doing, I think the issue I have had is just forcing the stretch specifically without lifting the whole car off the jackstands instead. I have so little weight in the car, it's just an empty shell and chassis. So I will have another go before separating that seam, but with a lot more ballast added. The way I've cut the A post and unbolted the front of the floor, it definitely is just the front floor/bulkhead seam holding things together.
On the passenger side I'm working on, the only thing I know to be correct for door fit is the door itself. It's a NOS Unipart one. Like you my plan is to get the rear door to rear wing gap right, then the front of the door to bonnet gap, then see where the lower gap is. It's impressive that you even managed to get the bottom door to sill gap correct with minimal fuss, most restorations I've seen still struggle with that one. But then again I suspect they're not paying as much attention to the A post position as you have.