I have one chamber to cut and will never do another one, I promise!

Post a picture of it;
(I just want to see one; I don't have a reamer)

Picture 460 Weatherby basic cut to 2.800" length with straight taper to hold a .510" dia. bullet

Ok; it's pictured in my brain. Here is what I would do. Get a 460 reamer.
Grind it so it will cut at the shoulder face. Put a .500 pilot on it.
Since the shoulder is about .560 and you need one about .535 or so, might want to grind it a bit smaller.
Cut the throat with a .510 throater; I will loan you that one.
Move the belt back. Or cut the belt off and ream the belt with another reamer. Or bore it.
Not hard if you have a tool grinder.

I'll let you do it.

An alternative way to more or less the same cartridge is the 500 Ultramag. Use a 50-110 reamer to cut the basic chamber, and then lengthen it as desired using a .50 N&T reamer. It headspaces on the mouth of the cartridge, because no belt on the ultramag. I've got both those reamers, to do exactly that. You fireform 375 ultramag with a load of cream of wheat to blow out the neck straight, trim to length and good to go. Can use 50-110 dies to size and seat.

Or I just do it with a boring bar and .50 cal throater

The .495 A2 doesn't straight taper to the case mouth. It has a ~0.400" parallel section or ghost shoulder that starts at ~2.400" from the case head.