Going to Johnny Mac's post he lists hard and fast requirements.
1: Stock port locations.
2: Stud mount rockers oiled through the pushrods.
I know valve length and installed heights. I went through this to install the LM7 valves in my 1966 289 heads.
As I mentioned, Magnum valves can be used. Pretty much all the OEM's are using beehive springs these days. My PAC springs were not cheap either. Now when Blue Print Engines orders up valve springs, they will be ordering a few sets more than you or I will ever use giving them a better price point. Now if they are building a bunch of Gen 3 GM engines and the same springs can be used the prices look better.
When designing a new head they can engineer in pretty much any installed height for the springs that work for them. They can specify valve stem lengths and head diameters to fit their needs and on their volumes the costs balance out a bunch.
Maybe Johnny Mac could comment a bit on this, with a vviewpoint from their end and considering input to this point.