Stunning pick up NorthGaAire - lovely work all over to say the least!
Well there is a flea's whisker between the old .303 & .405 Rim Diameter; but the important thing is that the .455 OAL will fit in a normal M98 magazine easily.
A sloping mag will most certainly help like the Siamese Mauser does.
With the Mauser its strength will certainly boost the .405 into a pretty powerful beast, not that the '95 Winchester is a slouch in that department either.
Hope to see what she'll do, don't forget your loading notes when you get a chance to do so.
Lucky bugger!