Trex,
In response to your question about the 458 Lott I would load about 83 top 84 grs. of IMR-4320 behind a 500 gr. Northfork, Woodliegh or Swift, and some Northfork flat nose solids, or just use his wonderfull Northfork cup points altogether...

In the 338 I would load up some 300 gr. Woodleigh PPs and about 10 flat nose solids...

If something went wrong with your 458, never think the 338 won't kill a buffalo with the above loads, it will in spades..I have shot several with those exact loads....In fact the 300 gr. 338 Woodleigh will out penitrate any 300 gr. 375 bullet...Thats 95% of the battle.