One point: The gun is not the same. They are ballistically equivalent, but that is the end of that point. They take different loads to achieve the same velocity. The brass on the 45-70 is more voluminous than on the 450, so using 450 data in a 45-70 should bereasonably safe. I would simply start the load slow and work up. Varget produces extremely accurate loads, but works much better when the case is nearly full. Using the type of bullet you indicate, you will be lucky to get that much velocity with any accuracy. I use the Gould, and love it a lot. My best hunting accuracy is around 1400fps to 1600fps with a 1700 doable. Varget with a full case on a 340gr. bullet will definitely go beyond that safely. I guess experimentation on this aspect should be fairly safe. I used 60 gr. of Varget for a Hornady RN the other day and got lower velocity than I wanted with a full case. Accuracy was very good. I would recommend using another powder. One possibility is H4198. Try out 30 to 36 gr. Another is XMP 5477. Love that powder. Produces very good results for a small volume load. Good luck and keep your velocities down for accuracy! My best accuracy comes from using Greg Mushial's pistol powder loads. One example is Titegroup at 10gr. Velocity is low, but is very accurate. Mainly a plinking load.
