Would have thought welding a bar across the donor strut tops would have been a good idea for when you come to put them on yours. It would make it a lot easier to get the geometry right. you could just position it 1" higher and weld both side on. Now you got to mess about getting it right on both sides.
You've made a easy job hard in my eyes
I think the only way you could weld it and be sure the geometry is all correct is to put the shell on a jig, not something you want to be A) guessing or B) lining it up be eye and thinking that is close enough.
Get it wrong and it will affect tyre wear and how the car drives and feels
Going to be all the cock I fear, seems complete madness to me to take what at the moment is a nice car and come very close to ruining it all in the name of scene points / parking your sump on the floor.
Sorry to be harsh but your taking a lot of strength away from the shell and yes he may know what he is doing but can he put all that strength back in.
It may also be worth speaking to your insurance company, they may ask / need an engineers report to confirm the standard of work.