Trail Boss works fine for reduced loads in the 45-70. For full power loads get some Varget or 3031, It is much easier to work with in this round, produces from 1200 to 1900 fps with the 405 grain bullet safely and excellent accuracy. I have never had much luck with 4198 in the 45-70 and find it is too fast a powder for the heavier bullets. Lots of pressure with low velocity.
Published loads for the Marlin with the 250 grain and the 405, which is close enough to the 420 you are using just work up from min. With cast bullets and no gas check, you want to keep your velocity under 1500 FPS to avoid severe leading issues. If you work up with Magnum primers you will find far less unburned powder left over, especially in the 18" barrel models.
http://www.hodgdonreloading.com/data/rifle