I have three words for using .450 Marlin data in the .45-70 ... NO NO NO!
You are dealing with two different cartridges.
I have a Marlin 1895 .45-70 purchased new in 1977. It will take some stout loads but the loads I've seen recommended for it the past 10 years give me the willies.
One load in particular that is frequently cited from Elmer Keith's writings --- 50 grs. of IMR3031 under the 405 gr. jacketed bullet --- is clearly too much pressure for my rifle.
Years ago I loaded a box, shot five rounds (examined the fired cases after each shot, which should be standard practice with every reloader using a new load) and never shot any more. I pulled the bullets.
Later, I determined that 50 grs. of IMR3031 was MAXIMUM in my rifle.
The point being: rifles vary greatly in their likes, dislikes, strengths and weaknesses. What is safe in one may be unsafe in another.
There are numerous yokels posting all kinds of "information" on the .45-70 with heavy loads on the internet today. ALWAYS check their information against the latest reloading manual, or a manual no more than five years old.
I've been waiting for your question to come up. Knew it was coming.
What so many reloaders forget, or never knew, is that each rifle is an individual. Also, case volume is not a benchmark to guide reloading. The seating depth of the bullet determines how much volume the case has at the time of firing, not total case length.
Forget using .450 Marlin data in the .45-70. And if anyone tells you they've done it, regard that person as a fool for taking such a risk.
Frankly, I can't understand all this hoopla about making the .45-70 into a Magnum. Factory loads, with their heavy bullets plodding along at 1,200 fps or so, have reliably killed game for over 125 years.
Just because X is good, doesn't mean that X-plus is better. Perhaps X is all that is required. When did our game suddenly become super-animals?
A classic example is the plethora of jacketed hollowpoints for the .45 Auto for self defense; nearly a century of use shows that the 230 gr. hardball bullet works just fine against human aggressors.
I've safely loaded my Marlin 1895 to 1,800 fps with a 400 gr. bullet but it's a real shoulder-spanker. Anymore, I load the same bullet down to 1,600 fps. And for plinking and target shooting, 1,200 fps is plenty.
With a proper load, bullet placement is more important than a couple hundred extra feet per second.