The Handi's will easily handle 50K in certain cartridges. The 308 is loaded to 52K CUP (not PSI) but many others are loaded to well above 50K PSI.
The 450 Marlin, according to my sources is SAAMI rated at 43,500 PSI. BIG difference from 50K.
The Marlin brass is quite a bit thicker and will generate more pressure with the same powder charge as the 45-70, but the 45-70 can equal it's performance in every way, and the Handi can very safely handle 450 Marlin level loads. The question begs, do you need them and can you take them.
Like I said, start low, work up and see if YOU WANT to go that high.
As far as comparing the pressures of the 223 to the 450 or 45-70, it becomes a matter of bolt thrust, or the rearward thrust created when to pressure pushes the brass against the bolt (or in the case of the Handi, the receiver face).
The larger surface area of the 450 and 45-70 generate far greater forces and the small head of the 223. Some have converted to 300 Win Mags from 30-06 barrels and stretched the frame in a few shots, to the point where the barrel was loose when closed.
The Marlin levels of 45-70 loads will not damage the Handi, but will still hit the 2K mark with a 400gr bullet.
As far as the Handi handling the 50K PSI it is coming out soon in the 500 S&W and I have pistol loads in my Hodgdons manual just shy of 51K PSI. It is my understanding the factory loads are just under 50K.