That is not a great powder, cartridge, bullet weight combination. I modeled 338 with a 250gr in GRT. You're maxing out at like 80% case fill before hitting pressure with H4895. That's pretty low, but H4895 is fine with that as Hodgdon specifies it specifically for reduced loads. I would not use a substitute in that burn rate though as performance may get erratic, particularly with starting loads that may be below 70% case fill. Some more optimal options would be RL16, RL23, RL26, and VV n555. RL16 would be my first choice since it's incredibly temp stable and you're not dealing with compressed loads like you will for RL23 or RL26. VV n555 seems to be a very accurate shooting powder and is temp stable as well, just not as much RL16 or RL23. They're all great, all hard to find, and all at the top end of price. I'd grab whichever you can get your hands on first and just roll with it.
For 458 Win Mag, if you're shooting a 500gr bullet, you're good with most powders in the 223/308 burn range. Benchmark, Varget, AR Comp, VV n133, VVn140, IMR 4064 all work with ~95-103% case fill at max, which is about where you want to be. I'd go with one of the first 3 as those are all made for temp stability and I'm assuming these are hunting loads (unless you're a maniac who loves plinking with a 458!)
