How about because the CIP MIP of the 3 1/3 .450NE is 50,914 PSI (Pizo)

and therefore the finished item will have to withstand proof of 66,188 psi.

The fact that it can be loaded to less is irrelevent as somebody else may attemp to fire it later with a different load.

So no " a good qualified and competent gunsmith." is going to want to do it.
Further it would cost plenty to rechamber and re-regulate (not withstanding the jack screws) by the time one has done all this one is stll left with a baiakial that one would not be entirly sure of. For a couple of grand more there are used entry level german guns built for caliber to suit.

The 45-70 Baiakial is a great idea and a fun gun I would think. I have one of their shotguns for many years. I just built a 30-30 DR on a jones underlever for a fun gun. Great things but IMHO they should be left in lighter calibers and a better gun bought for the big stuff.

Regards