You are going to want a fast powder. I use viht as consistently available.
Either N135 or N140 will work ok, I'd go N140 as will work will heavier bullets too. Looking at the vhit website its 35.6-39.3 of N135 or 39-43.2 N140.
Start with a load somewhere in the midpoint. The 58s are short so its unlikely you'll get to seat near the lands and have enough bullet to seat. But measure the COL of your chamber and see what it is.
I assume you are just looking for Minute of Fox so try a col ~2.580" and see how it groups. If you want to improve then try adjusting the col in 10 thou shorter and longer (if you can still seat the bullets). Groups of 3 should be fine.
If increasing powder then look out for pressure signs.
I would contact Kranks and get one of the loadall books for .243 as this will give you values for a lot of powders. Do you have to go so light? It'll be fast so potentially a barrel burner and won't be legal for Bisley should you want to shoot there. I used to shoot 70gr in my .243.