Do you know if it's possible to cast bullets for a 425 W&R?
The bullet diameter should be 435-436, and it needs a gascheck.
I can design the bullet on your excellent web page, but I can't find a
.436 sizer die (Stillwell has retired…) and I guess an 44
gascheck(mine is about 435) is to small and a 45 is to large?
I hear rumors that "Buckshot" on the Cast Boolits forum is making custom sizing dies.
Lee will make custom push-thru dies, with the usual warnings about Lee quality.
If you were to purchase a 0.436" chucking reamer from McMaster Carr, you could ream a 0.430" Lyman die with nothing more than a drill press (RCBS dies are hardened, last I heard). Then use emery cloth and/or valve lapping compound spun with an electric drill to hone a taper at the entrance. The original 0.430" knock-out pin would be undersize, and would leak grease, but it would nonetheless be usable.
A 44 check would not crimp on, but it would still shoot fine. The shank and check will obturate to fit the barrel, so it is not that critical for the check to be a tight fit. You prefer it to fit snugly so it will stay in place while you are handling the bullets at the reloading bench, but they all fit the same after the trigger is pulled.
This is no different than using 45 pistol checks in a sloppy 458 rifle barrel. People do it all the time.
Some people even load bore diameter gas checked bullets in muzzleloaders. The undersize check obturates to fill the grooves.
I am sure that a GOOD sizing die could swage a 45 pistol check to 0.436". I haven't done it, but I have done similar things, like sizing 32 caliber checks down to 30 caliber, sizing 416 checks down to 41 caliber, 375 down to 9.3mm, etc.. One commercial caster sizes 50 caliber checks down to 500. The catch is you need a GOOD die with a large, gently tapered entrance. GOOD dies are scarce.