As an owner of a Marlin 1895, a Marlin 1895 Cowboy, and a Marlin 1895 Guide Gun (current one is not ported, the first one I had WAS ported), I stand by my statements. The recoil between all 3 barrel lengths that I have shot for the Marlin 1895 is relatively similar. I get about 2100fps out of my Cowboy with the same load delivering 1790fps out of the Guide Gun, but the extra muzzle weight seems to eat up the extra energy nicely...
In my experience, there is not a load that experiences enough change in recoil from one barrel length to another in the .45-70 that would be a tipping point to be NOTICEABLY different as it would be compared to a .308ME. A stout load will kick harder than the .308ME with an 18" barrel the same as it will with a 26" barrel. A factory load will still be a pup with an 18" barrel, as it will with a 26" barrel.
The .45-70 in factory loads, including the "original factory load" 70grn BP under a 405grn gas checked hard cast are actually quite mild (in deference to the SAAMI standards that are STILL LOW POWERED meant to prevent catastrophic failures in the older springfields that are still floating around).
Guys get so hung up on the punishing loads that guys hand-roll for their .45-70's and they forget that the factory loads are an entirely different animal. I hand roll the 325grn Hornady FTX for MY 1895's, while my wife uses the factory stuff for her guide gun. The same bullet leaves HER guide gun at 1790fps, where my handloads will push 2250fps out of it. I get 2510fps out of my Cowboy with this load (great bear medicine, but he11 on your shoulder). This isn't as hot as I can make these loads, but that load has been a "sweet spot" for accuracy and power for me.