Ah, what the hell - I think these "money no object" dreams...

Rifle: Holland & Holland Royal de Luxe
Engraving: Bulino game scene with Rose and Scroll done by the Brown Brothers.
Caliber: .500/465
Stock: Exhibition grade Circassian Walnut with deep, dark mineral lines
Options: Extended trigger guard and stock medallion - no extended top tang.

I would let the experts at the gun shop dictate the length of the barrels to ensure it is balanced perfectly.