Wyrtwister; You are correct about the barrel/reciever juncture being the weak point on the 45-70 Marlin. Thats why sticking to standard top end loads (for the Marlin) is a wise choice. Put a 45-70 in a strong single shot or bolt and its a whole nuther ballgame....
As most of you know, I modify 444 Marlin leverguns for more performance, and this issue with the 45-70 has kept me from working on them....other than action/cycling improvements. Mic McPherson modifies the threads on his 45-70's to accept heavier loadings, and that is always an option for those seeking to hurt themselves even more. Another option is the Winchester in 45-70....it can take pressures above what the Marlin action can handle, and is a heavier rifle that reduces some of the felt recoil.
In my modified 444 Marlin (the Peashooter @ 6.2 pounds) with my top loaded 325 LCMNGC Beartooth, the recoil factor is... 46 ft lbs at 22 fps. With my Safari Grade modification in a 7 1/2 pound rifle, the recoil factor will be about the same with a 405 grain Beartooth bullet at 2150 fps. In a full sized 45-70 Marlin (@ 7 1/2 pounds), the recoil factor of a 500 grain bullet at 1650 fps... is 39 ft lbs at 18 fps. Believe me, I feel your pain when it comes to recoil!
Perforator is right on by tailoring loads to suit the purpose. In my 444 Peashooter, I also use a 300 grain bullet at 1850 for deer and it works very well. For anything larger I can increase bullet weight and velocity. up to the top load with a 325 LCMNGC Beartooth bullet which produces 2263 fps and 3600+ ft lbs of energy. Utilizing the 45-70 with the best load for the job gives enough power to take care of business, and saves the shooter the abusive effects from recoil with loadings that are just not needed. IMHO shooting the hottest loads that you can in a 45-70 (or, any other caliber for that matter) when they are really not needed, only serves as a waste of reloading money, rifle longevity, and shoulder meat!!!! If you load for what you hunt, in the long run you will be a much happier, and less abused shooter. I always have a number of "specific purpose" loads worked up for each rifle or handgun that I use, but only use what is necessary to do the deed.
The nice thing about the 444, 45-70, and 450 Marlins is the versatility that they offer, and its a shame not to take advantage of that. To have one gun that can be utilized on any medium to large game animal by varying the load (and comfort level of the shooter) is a real plus in owning one of these big bores.