Your idea of using Hornady cases carrying the .275 Rigby headstamp is nostalgic. Read more
My advice is to sight in 3" high at 100yd. with the ammo you are using.
The .358 Norma will do nearly anything the longer .375 H&H will do and in a standard length action.
The warmth of wood and individuality - something traditional hunters simply hate to trade off.
The "name "Sun Lion" was used to describe the receiver crest on the Persian Mauser made by F.N in Belgium.
The inner shock wave from brake escape flows back and assaults your eardrums and may eventually result in hearing loss.
The .32 Special barrel would stabilise the heavier bullet and would also be better with cast bullets because of less rotational torque.
Even a heavy roundnose bullet from the .458 Win. Mag, will be deflected by twigs and light branches.
Make sure the bullet is a controlled expansion design since your shooting could be anywhere from 50 to 400yd.
You can figure on a reduction of somewhere between 180 to 200 fps with the 7mm Rem. Mag. fired from a 500mm barrel.
Button rifling causes the metal in the bore to displace around the die and is usually much shallower than conventional cut rifling.
Over time I've used slow burning powders in my .240 Weatherby, but always with a Magnum primer.
All .257 Magnum rifles built before 1973 had a rifling twist of 1:12" which is too slow to stabilise spitzer bullets much longer than those weighing 100 grains.
Your old load of 62.5gn of Re-19 would have driven the 90gn Sierra H.P at about 3500fps