Yep case length is important when using the standard roll crimps most die sets provide. If you use the Lee factory crimp its not quite as important unless a case stretches quite a bit - then you need to keep an eye out for case stretching near the head (base). You might want to hang out on the Marlin Lever gun section of GB for some info on cartridge length etc. I don't load 45-70 but a lot of others. I would expect reloading 45-70 for Marlins is darned simple just following basic directions - don't over worry the process. Handloading can be a detail fanatic's dream - there can be no end to it for some. But you don't have to get to that level for good basic ammo.
Oh and a case lube I like better for every day use is Hornady's Unique in a a little container like cottage cheese tub. My tub has lasted many years. In my cool climate and basic sizing I find it easier to use than Imperial Wax. Its a bit softer and wipes on the cases easier with my fingers. Wipes off pretty easy with paper towel after sizing. And you can oil your leather with it - it smells a lot like some brands of Mink oil - very mild smell.
Oh, one thing I do when I get new or new to me size dies is take a Q tip with a little lube and wipe the inside of the die well. My only near problem with cases getting stuck was with new dies.
And keep your cases clean before sizing - will keep your dies working better, lube works better.
With the Hornady lube: I put a bit more lube the the first couple cases in a batch, run them thru to get the die re-lubed. Then I dab my finger tips into the tub just often enough keep just a tiny bit of lube on my fingers, grab a case, roll it with my fingers set it in the shell holder, slide my fingers down the case body to the base then run it thru the die. Some times I drag a finger tip around the mouth of the case if I think the inner case neck needs a bit of lube. But mostly I graphite my inner case necks with Imperial's (Redding now) graphite lube. But that's one of those details that a beginner doesn't need to mess with, especially on a straight wall case. I mostly load bottle neck cases, 223, 30-30, 30-06, 300 win mag, 348 win when using lube. I use Carbide dies for all my straight wall handgun ammo, 9mm, 38/357, 41, 40.
Remember you don't want lube in the primer pocket or much on the inside of the case where it might contaminate the powder or let the bullet slip. A tiny bit the inside of the neck makes it easier to pull the case off the expander ball and may keep cases from stretching. But that is probably more noticeable in bottle neck cases.
Again, I think you need to get a loading manual and read about the whole process.
