Hi there! A couple of questions back to you for your consideration. What distances are you planning on shooting the rifle? Like modern rifles the sites are important out to longer distances, especially with a 45-70. Tang Mounted Precision sights are recommend out past 200 yards so you can dial in the bullet drop out further. Our long range set up at Green Mountain Regulators has 2 courses of fire, one for Precision sights and one for barrel mounted factory. Barrel mounted sights go out to 200 yards and our Precision goes out to 400.
A 45-70 depending on bullet weight, speed and design will have very different results sometimes. The original cartridge was designed for a heavy bullet (500 + grain) although you can get a lower weight ( I think I saw a 350 grain somewhere). For your Henry, the 405 may be the best general tip to use, a heavier one will have significant recoil due to the weight of the gun. It is also a good choice out to around 300 Yards, it can fall off in consistency after that due to environmental factors. I have had good and bad days out past 300 with the 405's. I have shot my rifle out to just over 600 yards with a heavy bullet and it seems to like the heavy ones the best.
I shoot a Rolling Block pattern rifle with a heavy barrel and use a heavier bullet, 485, 500 and 525. Find some loading data that will push your speed to somewhere between 1100-1250 FPS. I have found that mimics the design speed of the old BP cartridge and has had very good consistency with 405's to 525 grain bullets. Keep in mind the ballistics of the 45-70 is a lot like artillery, for example a 405 gr bullet at 1250 fps will drop about 164 inches at 400 Yards, with a zero at 100 yards. Every rifle I have had likes different loads for some reason, I have tried Unique, 5744, 4227, and 4198 and different charges till I found one that is consistent.
With a 405 grain bullet, a few of our shooters use Unique at around 14.2 grains, it is a soft shooting load and I think is around 1125-1150 fps out of a 26 inch barrel, out of a shorter barrel it may be different. It might be a good load to shoot with the Henry, I have shot it our of a Marlin and it wasn't too bad from a recoil perspective. I recommend looking for older Lyman Cast bullet loading manuals they have a lot of older data that may be very useful in developing a load. My favorite is the 4th edition.