Jolly good show!
I have Lee dies and use a RCBS RockChucker with the standard die insert removed (allowing the BIG dies to thread directly into the press). You might consider the relatively new Lee Cast Classic ... I'm certain that their big dies should fit ... more on that in a subsequent post.
DISCLAIMER ... any loading data posted is safe and works for me, you use it at your own risk.
I shoot smokeless loads ... 36.0gr of #5744 with a 450gr bullet sized to .451" and paper patched to .460". Bertram brass. Winchester LR primers. (My MkIV uses a 480gr bullet sized to .453" and paper patched to .470" with 34.0gr of #5744).
I would try chambering all brass first, without resizing. If everything fits ... just worry about the neck. With subsequent firings/reloadings, I just leave the neck alone ... I can thumb-seat my bullets.
I always make certain to store my loads bullet up and when chambering for a shot, I elevate the barrel so the powder stays at the bottom of the case. I have NOT experienced hangfires or misfires or anything out of the ordinary and was surprised how accuracy got better and better over the first couple of shots. (The paper patch was scouring the grooves of lead and crud and after about 3 or 4 shots ... nice round holes just about where I wanted them to be).
I have read about guys neck sizing with .458 Win Mag die or a .480 Ruger (IIRC). I haven't tried that yet. I think it is important to never overwork the brass ... just too expensive. If you can neck size just enough to hold the bullet ... that's what's best ... hence thumb seating if possible.
I hope that some time this year my gunsmith gets one of my Martini Project Rifles done. I have a number of distressed but otherwise sound Martinis and want him to take one and build it with a .450" bore/ .458" groove. Then I can shoot either paper patched .450"/.459" or a host of .459" grease groove bullets, but still be in full military condition ... though I will probably get some aperture sights mounted on it.
My caster died and I don't want to mess with casting and the cost of "Martini" lead gets expensive ... so I just put the money in a rifle instead.
Hope this was of some help. I've probably out 75 rounds downrange with this combination and it sure was fun!