Your 7mm-08 oughta work just great.

But... if you have to have another rifle (), I'll be the killjoy & bring up the dubious science of caliber spacing. A .30-'06 would be a superb choice, but it's not all that much different than your current setup. Maybe check out the .35 Whelen, as KonaBoy suggested, or .358 Win., or .338-'06, or .338 Federal, or... something bigger than .30 cal.

Naturally, there's always .338 Win Mag, but that's lots more than you'd need for blackies, & you're looking at substantially more recoil, muzzle blast, & ammo expense. Then again, none of the calibers I mentioned above would be exactly "cheap" on the ammo expense scale, owing to their less thanwidespread popularity.

Another thing to consider is range: the calibers above would be good 200-ish yard cartridges (generally speaking, folks!). So, if you want a "reach way out there" setup, then you'll likely be looking at a magnum of some kind if you're going with a larger diameter bullet.

Good Luck,

FC