Camp Cook
Speer #10 had numerous loads that were reduced in later versions to worn test barrels producing lower than typical pressures. I dont know if that affected the .45-70 or not, but I do know that I have found several errors in the Speer #12 and reported them to Speer who provided me with corrected data.
Speer #12 lists the following load:
Speer 400g @ 2,540, 49.0g H322, 1652fps, 28,000 CUP
Speer 400g @ 2,540, 60.0g H322, 2016fps 35,000CUP
Hodgdons Annual Manual lists the following:
Speer 400g @ 2.540, 55.0g H322, 1984fps, 39,2100CUP
I also started out with Speer #10 back in the early 80s and have had to reduce my 7mm Mag 160g loads accordingly using the #10 data case life was very short (4-5 reloads). I backed off the powder and now case life is up to 18 reloads, and the chrono I acquired several years ago shows velocity is where it should be. My new loads pretty much match Speer #12 data.
I cant say what pressure your load is at but it is highly unlikely that it is in the 28,000 CUP range, even in your unique rifle. More likely its in the area of 40,000-45,000 CUP based on the Hodgdon data, which I trust more than the Speer data. A year or so ago I read an article that showed that in different cartridges a 3.0g increase at already max pressures could bump pressures up by nearly 10,000 CUP a very good reason to be careful when working with max or near max loads.