I advise heavily against it but it could be done, but you would have to have a reamer made that would allow the longer case to headspace on the case mouth like a 45 ACP and let the belt float. So long as you didn't want to wreck the gun, you could load it just fine at pressure levels of the 450 Marlin. This might gain you a 150 maybe even 200 feet per second over the 45-70, so I see little point in it.
To get an idea of what is really being talked about, it takes 45 grains of IMR 3031 to drive a 320 grain bullet at nearly 1400 fps from my 45-70, the load generates something on the order of 14,000psi. My 44 Magnum drives a 300 grain bullet at 1550 fps with 22.5 grains of H110 but requires a pressure around 40,000 psi to do it.
Conversely, a 45-70 will drive 400 grain bullets at 2000 fps at pressures of 36,000 psi, the 458 drives the same bullet at 2400 fps but takes in the neighborhood of 52,000 psi to do it. The 416 Rigby drives a 400 grainer at 2400 fps with only 42,000 psi. The Rigby also has a mystique and reputation the Magnum Winchester or the Lott do not have. I would set my sights on a Rigby, but it can't be done in a Handi.
