RE: Do I need a .338?
No offence there cma3366a but the 45-70 is far from a deer and black bear only cartridge and I certainly wouldn't consider it a short range number either. People have taken large game animals such as bison from distances of 1000 yards with the 45-70 with a single shot. A 30-30 is a short range elk cartridge, the 45-70 isn't even in the same ball park.
The 45-70 is up to the task of tackling the worlds biggest and most dangerous game animals when the correct ammunition is chosen which is readily availble from companies such as Corbon, Garret, and Buffalo Bore.
A 300 grain, hard cast, flat nosed bullet leaving the gun at 2350 fps will flatten any animal that you will encounter on this continent. If sighted in correctly it will have a MPBR of +/- 3 inches to 225 yards.
I would never feel at a handicap toting a 45-70, 450 Marlin or 444 Marlin when going after any animal in North America.
Having said that, the .338 Win mag will have a definite edge in the trajectory department but certainly not in the killing/ stopping department.