A Lee collet die for a 300 Weatherby should work but the seater and FL sizer would be a custom proposition. Expensive unless you already had the reamers. With the venturi shoulder your on the "C+" list of special dies.
Probably less expensive to set the barrel back and go 300 Weatherby but then you've erased the history and nostalgia of the rifle.
What you think you're seeing is the original crease in the brass from the the H&H round.
This is also evident when I fireform 375 H&H brass to 375 Weatherby. It disappears with a full house load.
Donuts only occur when using bushing dies. My 338-06 has a definite measurable 'donut' at the neck when formed in the die, after firing, it's gone and doesn't re-appear.
Cheers.
