H322 is a relatively slow burning powder for the 45-70 and as such does not lend itself well to light loads. Even with full power loads it often leaves visible powder residue in the bore. However, it is similar to IMR3031 and I have used 35 grains of IMR3031 with 485 grain Schmittzers in my Buffalo Classic. You could try working backwards from 40 grains to see if you can approach your target velocity without experiencing ignition problems.
