You WILL NOT find a consensus on what pressure levels are 'max' for the various .45-70 'heavy' loads.
Read several different loading manuals, seems like everyone has their own limit/tolerance. Hodgdon, Hornady, Speer, Sierra, Lyman, and others all have suggested data.
However, some do list the Win 1886 as suitable for the same 40,000 CUP loads as the Marlin. Makes sense, if anything a modern 1886 should be able to handle more as it was originally chambered in some 40,000CUP smokeless rounds with the same case head size as the .45-70. In fact, the 71 Winchester is basically an 1886 action, but for the .348 Win which is actually a bigger case head than the .45-70, again in the neighborhood of 40,000CUP.
Probably the reason for the difference listings is that even with a good strong design, some of those 1886s have been around since, well, 1886, and metalurgy wasn't as exact back then. Plus who knows how they have been treated. So be cautious with original 86s.
But a modern reproduction should be about as strong of a lever gun as you can buy.