I've shot cast in both calibres since the early 70's. In the .375, using LBT Blue, I went all the way to 2,740fps with the .375 Lyman gas chekced mould, sized .377" without leading, using hardened and tempered WW bullets to about brinel 20.
: In the .458, I've shot several bear and moose with cast bullets in the 340 to 560gr. weights, from 1,650fps to 2,100fps with the 340gr. and 1,960fps with the 560 gr. Lyman, all in straight unhardened wheelweights. I've never had leading problems in 3 different .458's using WW alloys as long as descent lubricant and .459" bullets are used.
: The nice thing about the .458, is that straight WW alloy, if cast and sized oversize and a descent lube is used, like the Lyman Moly, the alloy will take up to 2,200fps without leading and expansion is pretty much gauranteed, without breakup that happens with harder alloys.
; .458's are easy to cast & there are a lot of good moulds available in that calibre. My current .375 mould is the RCBS 250gr. Mine runs 265gr. in WW alloy and shoots splendidly sized at .377" both in the .375 Big Bore '94 and .375/06IMP at up to 1,900fps in straight WW alloy. I haven't tried hardened WW bullets in it yet.
; Straight WW alloy will harden up to about brinel 33, which is 50% harder than Linotype, but not as brittle, so it still penetrates fairly well while allowing higher velcities.
; Shooting lead in the bog bores is fun and allows much cheap shooting. Most powders work well in the large ones as well, with H4895 being one of the best for powerful loads and down to about 1,600fps too.
; Lyman's cast bulet handbook is a good one to get, with some good articles in the front on cast shooting as well as much needed general info on shooting lead.