325 grain bullets in .458 are an oddball size so I have never seen them even though I have used .458's for decades.
You should be able to use 350gn charges as starting loads and maybe get another 2 or 3 grains to suit that weight is you are stuck on it.
Rel 7 should used around 69gn and possibly a couple more grains
H4198 is a good choice but I'd start with 75grains. There is huge variation with the recommended powder charges for this powder and I have see 10 grain variations but because I have loaded a damn lot more than most with this case, I know the charges will permit easy extraction and no signs of excess pressure in this case.
John
