Some of it depends on your rifle and the O.A.L. of the cartridge. Make up a dummy round and see if it will cycle in your rifle with the bullet you wish to use.
And yes with any new load you should work up the load.
Or you can see if you can trade your Hornady brass for some standard length Remington brass. You may fine someone on this forum or the Marlin Owners forum that will do that.
Or go direct to Starline Brass and order there 45-70 brass. Great prices and fine brass.
As for choice of bullet I was not happy with the accuracy of the 300 Gr bullets from my Marlin 1895GS in 45-70. But it did perform much better with the Hornady 350Gr RN of 3/4 and less at 100 yards with a scope.