Rabon,
I have used the 400 grain Cast Performance bullet, and I am able to crimp in the groove. You will never want to use any type of roll crimp with the 458S. It will likely ruin any chance od the round chambering properly. With my cast loads I apply a little taper crimp. Not excessive, but enough to ensure that the case mouth bears a little on the top of the crimp groove, to make sure I do not get bullet set-back when feeding. The Speer bullet should work in the crimp groove. I have played with other Speer weights that did so. Please bear in mind that a crimp in the 458S is not mandatory. Likely it will only wreck accuracy.
The critical component of the crimp theory in the 458S is whether the bullet sets back in feeding. Revolver crimps are designed to keep the rounds from creeping forward in the cylinder under recoil. No issues here with the 458S. Just feeding setback that likley will not jappen. Make sure the neck tension on sized brass is good, do not excessively flare the case for seating, and likely toy will be good to go. Just need to test some dummy rounds for feeding setback, and go from there. I only really crimp my lead loads, as the noses (meplat) are so darn big I want a little insurance.
Green,
I say try the H110/296 load as it is very easy to work with. Then, once you have the reloading down, and want to try others, go for it. Again, I am not saying H110/296 is the only way to start, just reasonably fool proof.
I am not sure what to make of the Barnes info. If it were me I would 1)call Barnes and ask to talk to a tech, and 2) do a little penetration testing on newsprint or something. Even if you have to download a bit to get slower velocity for up close testing. I would say categorically if Barnes says the bullet is designed for 45-70 use, then I would likely not worry much about the expansion. If they say 458WM, then I would certainly test, no matter what they say.
The only 45-70 spire point that I am aware of is the Hornady LeverRevolution bullet. But, I do not think it is available as a component yet (perhaps never). One bullet that I had really good results with as far as accuracy is the Barnes original. It is technically a spire point with lead tip. I never tested to see if it would open at 458S velocities, as I only had a couple of boxes, and only got as far as velocity accuracy testing. They only run these bullets a finite number of times a year, and they can be hard to find, sometimes. Might be worth a look if you can find them, and want to test.
I have not heard that W296 is discontinued. I would find that to be very surprising, as it is a very popular powder.
Craig