Ian, this is a Z body-shell dilemma that lots of us face. I think that your problem is compounded by the fact that you have already spent a lot of time and money on this car and you have met the problem at the end and also the scale of the problem was unexpected.
Yes, if I had known about these issues before spending the money on this shell I would have re shelled.
You are a great engineer and a perfectionist so you want everything to be 'right'. That is why you have gone looking for body issues and these Zs can bite back. Derrick (candyred) bought a 260 that I'm sure he thought was reasonable but on inspection was far from it. However he is repairing his car himself and you are having a professional job done - hence the money issues.
I would love to be able to do more work myself, but my situation just doesn't allow it. I'm hoping in the future to have another long term car project which I will be able to do gradually by myself in my garage. Its unlikely to need as much metal work as a Datsun though.
I've talked with Dave at Resto Shack today and it looks like we will be repairing this shell, given the cost to get a new shell to this stage it doesn't make sense financially and Dave isn't worried about being able to repair it, only part that will be a test for him is the RH quarter section. Metal work won't be starting until early Jan probably as they hadn't planned to be doing so much. If anyone knows where a decent RH side quarter section is available then let me know, I think its unlikely but I have seen them come up before the odd time.
Since there is so much work going on I am considering going bumper-less on the rear and fitting custom rear arches, will still look close to stock but will allow a slightly wider tyre.
I build my cars for me and don't plan to sell it, plus its not exactly an original car with the RB anyway so I'm not really concerned about opinions. A lot of people seem to worry a lot about value, but for me if you do this too much then it takes away the passion of a car project, I try and leave the worry about values to my daily driver cars.
You can be sure I will leave the metal work in as close to perfect as possible, and by the time its done it will be more structurally sound than when it left the factory. The photos taken throughout the restoration will be proof of this.
If you import a very good, late 260 shell from the states and convert to RHD it might be about the same cost but will be a 'nicer' and better shell under the paint (but converted). It should be worth about the same as your car repaired.
Lets see how Resto Shack leave it, you might find the shell will be pretty good and it will certainly be much better than a lot of the cars out there that could be hiding unknown potential problems.