I have four different Lee molds 340 to 500grns. I enjoy the 500 the most, however I don’t use as much powder as for a 405. The new Marlins are a lot tougher and can take a lot more pressure for sure than the old trapdoors. However, they are not the strongest .45-70’s built today.
If you really want to go for the high speed, you are probably better off with 340grns. Maybe a faster boolit will shoot a little flatter, but there is always a trade off. A heavier boolit will have better knock down power, especially at a greater distance.
The Lee molds are cheap and you can afford to experiment with several sizes. Montana bullet works sells cast boolits from some of the more expensive molds. There are probably other casters who sell similar boolits too. Best to try before you buy so you are happy with what you get.
https://www.montanabulletworks.com/