The H&H is only .030" longer than the Weatherby. If I were to do this in my shop to my rifle, I would make a Cerrosafe cast of the chamber, then have a 300 Weatherby reamer made that was .030" longer than standard with the neck diameter the same diameter as the 300H&H chamber as measured by the cast to make sure that the new reamer would completely clean up the old chamber. The new reamer would follow the old hole. No barrel set back would be required.

A 721 will handle a 300 Weatherby just fine. I have read about a lot of guys having old model 70's rechambered from h&h to Weatherby and I don't know if they set the barrels back or make special reamers. I don't like the idea of using .030" of the old existing H&H chamber since it may be larger than what the new reamer would cut creating a reverse undercut of sorts which may or may not create an extraction issue depending on what the exact dimensions are and how much brass spring back you get. Personally, I would have a hard time spending this much time and money on a factory barrel. I would just rebarrel to whatever belted magnum I wanted with a quality aftermarket barrel.

I don't understand the concept you mention of rechambering with a 300 win reamer and then lengthening the neck with an 06 reamer. You would still have to set back the barrel to do that (if you run a 300 win reamer in a 300 H&H chamber you will still have some of the long tapered shoulder of the h&h remaining - a 300 Weatherby will clean up an h&h except for .030 of the neck). Then you have to form brass from 300 Weatherby or H&H. I would think if you went to the effort of setting the barrel back so that a 300 win would clean up the h&h you would just keep it a 300 win. Maybe there is a feeding issue with the 300 win in a gun originally set up for 300h&h and that is why someone suggested the "long 300win concept" I'm not sure.