Don't let anyone deter you from shooting cast with a microgroove barrel. What's important is the bullet size. Either slug the barrel or just go ahead and try .460" fodder.

That 22" barrel will get you to almost 2000fps with a 405gr cast. There are so many good powders with the 45-70 that I hesitate to get dogmatic about a load. I really liked Benchmark, but I also liked H322 and just about every powder in Hodgdon's extreme line. I've settled on H4198 - if you want some cast load data pm me.

When you have shot 3, check the muzzle for a lube ring. If you've got one then the bullet is sealing just fine. If you don't then the bullet is likely undersized (or you don't have any lube )

My guess that you'll be fine with .459 -.460, iffy with .458" and pathetic with .457". I don't know the art of casting so I don't know what your mold throws.