There's no problem using the full-length 450 case and necking it to whatever. Or trimming a Win Mag case to 450 length and necking. You can blow up any rifle in any chambering, by using the wrong powder. You need to understand the limits of the action, and keep it well under, in this case somewhere under 40k psi.
Figure something a little less than 7mm-08 in muzzle speed, and then figure the lower BC and greater drop of FN bullets, and that's about what you're describing. Maybe 2700fps with 140g bullets.
As for case capacity, bigger is better in this case. The more powder you can get behind the bullet, the faster it will go. For a low-pressure load in a lever gun, you need a slower powder generally, than in a bolt-action, say. But in any given pressure range, the more powder room, the faster you can make the bullet go.
To wit, use a 404 J case, trimmed to the right length and it'll go faster than this H&H case.
