Dale,
since getting my 1895GS I have learned something about the Marlin 450. For one thing the rifle is stronger than the 45-70, in so far as the threads on the receiver are a different design than the 45-70. It's the same design as the new 308 Marlin Express uses. I learned this after asking if a 336 could be converted to a 308MX and was told no, and the thread design was why. To my knowledge, the receivers are the same otherwise.
The newer design is stronger.
If you look at the specs on the Hornady Leverevolution ammo and compare the 45-70 to the 450 Marlin, you'll notice about 150fps advantage to the 450. It's loaded to higher pressures than the 45-70, thus the difference.
If you reload, I'd actually suggest the 450 over the 45-70, because all bullets, in the Marlin, are cross compatible, and should you desire to punish yourself, you can most likely load the 450 to a slightly higher velocity.
You cannot, however, get a 450 Marlin guide gun in stainless, should that be a desire. No one seems to know why either.