I would think your RL 15 and IMR 4064 would be both worth trying. If in fact the 358 WSM has the same powder capacity as the 35 Whelen then very likely Whelen loads would be a good starting place. Not much sense in me suggesting other powders that you don't have in today's marketplace but Look at the new Speer data for use of some new powders in the Whelen. When you can buy powder again, CFE223 and some of the new Alliant powders might make your rifle sing.
