Gentlemen:
Yes it is raining but not on my parade!
...but thank you for your concerns.

Firstly, it is not my original idea to use the 300 Win Mag. I found one other report on a web forum describing successful use of that case as a donor.

I am aware of the potential suitability of using the Ruger cases, but necking 375 or 416 down to 280 is not without its price: annealing (twice!) is a given, but neck-turning or reaming would be crucial, and necks would probably crack before long anyway despite best efforts. One handloader reported cracking the sizing die during case forming!!
...and speaking of price, have you bought any 375 Ruger cases over the counter lately?!!

I initially tried a couple of 7mm Rem Mag cases that were lying around, intending to spin the necks out on a mandrel in the lathe to get them to the proper length, but they were just too short to make this viable.

Here's a couple of pics to allay your fears:

Left to Right: .300 Win Mag; Formed 280 Ross; Fireformed Case; Original Kynoch 280 Ross

Loaded round ready for fire-forming, alongside a Kynoch original.

Close-up of the head of the case after fireforming.

When a straight-edge is placed along the tapered body of the case, it just touches the front of the belt. All good!

...and the Ross M-10 extractor claw is plenty big enough to extract the case and hold it on the bolt-face till it hits the ejector.

Will try to grab some brand new 300 Win Mag cases this arvo to start afresh. This is fun!

BTW ExcessExpress, any hints on why there is a chook on the grip-cap? Hardly a grouse-gun...!