Hi msorenso,
CIP is the european standard for dimensioning cartridges incl. specs for max pressure and test procedures. Every single barrel of every gun is proof tested with 30% over the max. pressure given for a specific cartridge.
I don´t know what FFL is, but I know that the american pendant is SAAMI. As you live in the US, you don´t need CIP proofing.
The 577 NE fires a 750 grain bullet at ~2050 ft/s. It started as a Black Powder cartridge. With the availability of Nitro powder (in that times cordite) the cartridge shell has been beefed up and a extremly powerfull cartridge was the result.
The 600 NE fires a 900 grain bullet at about the same velocity, but was yet designed for Nitro powder.
The 600 JDJ is a propriatery cartridge of SSK. He uses the same 577 cartridge shell, straighten it and fires a 600 NE bullet.
The problem with the Ruger #1 is the action size.
The barrel needs to be strong enough to withstand the generated pressure. The chamber for such a big cartridge will not left much material around the area where the barrel is threaded into the action. (see my pics). You will need a barrel made out of the best steel available.
For me the 577 is really all I need and is one of the best stoppers in the world. I tell you my gun could be a bit heavier as it kicks like a mule. ;-)
I understand that you want the biggest caliber at a reasonable price, but it is IMHO a good advice to shoot some of the bigger cartridges before you start.
br Tom