I have a Ruger No.1 in 45/70 that I'm considering having rechambered to 458 RCBS. My understanding is that this is simply a 45/70 lengthened to 2.75" and is made using 45 Basic brass.

I'm wondering whether it is still possible to fire normal 45/70 cartridges in the 458 RCBS chamber? Just like you can fire 458 Win Mag in a 458 Lott chamber. Would this have any major effect on accuracy?

RJW

Why not rechambering it to something more impressive like the 450 NE No2 or a 460 Weatherby?

I am not sure why would anymore want to delibery shoot a shorter cartridge in a longer cartridge's chamber. A 458 Lott rifle can shoot the 458 Winchester but only for emergency use in case 458 Lott ammo is not available. For the peace of mind, it is not a good thing to do this, especially with a high pressure cartridge. Good luck!

RJW- The 458 RCBS is probably not a bad choice as it's a simple re-chamber job on your Ruger#1. If you get a reamer made without a rim -cutter( or buy one and grind the rim cutter off, it would be possible to do this re-chamber without affecting your current headspace. While you could do this yourself, having a good gunsmith do it for you is not a bad idea as you really need to use a Lathe so as to not ruin your chamber. The 458RCBS is basically the same as a 45-110 with smokeless loads. You can go all the way to a 45-120 and probably get pretty close to 458 Lott ballistics. That's gotta hurt out of a Ruger#1.
As for shooting 45-70's out of it. The only danger is that with continuous use( read-a lot) you might cause a rough spot in the chamber that may effect your extraction of the longer cases. I do this all the time with a 45-90 so don't sweat it. Don't expect great accuracy though.-Rob