Bramble:
Here's how I look at it: use the proof data to help you choose an action, but be sure that it is well-built, obviously robust in construction, has a third fastener preferably a cross-bolt, and is made with (relatively) modern steels. Then when the barrels are fitted, fire a couple of proof loads 30% above the mean working pressure of the chosen cartridge. If it passes, it is now 'in proof' for the new load, regardless of what it was proved for before.

There are several ways to estimate a 30% overcharge, contained in previous threads on this subject. As I recall, most employ an over-weight projectile, rather than over-charge of powder. A search should give you a starting point.

Like hell! Many of us here in Oz are very leery of Reloader powders for the big nitro expresses. I know Seyfried and others have popularised them in the States, but neither I nor any of my close friends use the stuff, being unashamed AR2213 or IMR4831 fans for the NEs. Filler is usually not required, but a felt wad or tuft of dacron would probably be needed in the over-sized .450 No.2.

If you match published factory velocity with AR2213 or IMR4831, you will invariably be well below the pressure for which the rifle was originally proved. All modern published pressure tests support this assertion.

BTW Bramble, I would give serious consideration to disabling that 'both barrels' feature when converted to .450 Nitro!