Sorry for the late reply but I just noticed this topic. I have the same rifle as you (a Ruger MK II in 350 Rem Mag). You have received some excellent advice from the other posters. I'll tell you what I've tried on my rifle but be forewarned that trying to tell someone what scope to put on his rifle is akin to telling him what type of woman he should marry. Only you can make that decision. Ok, with the disclaimer out of the way, I've tried an IOR 4X with the 7A reticle and a Leupold 6X FX III (not the new FX3) with the Heavy Duplex reticle. I am currently using the 6X Leupold. It has great eye relief, works well enough for hunting during legal hours (at least where I hunt) and I can instantly obtain a good sight picture (which helps tremendously with the somewhat limited FOV). With this scope I've had no problem taking successful fast shots at deer, which travel through a narrow shooting lane. Regarding toughness, I accidentially dropped an unloaded rifle with the Leupold 6X out of a 15' stand. The scope was dinged but it still held near its zero and it works fine to this day. So, for me, this scope works well. I did try a Nikon Monarch 2.5-10 X 42 on this rifle but I just didn't like it. I had gotten used to the Heavy Duplex reticle and the lighter weight of the 6X.
One other thought: Since you are used to fixed power scopes (particularly 4X), it may be a good idea to stick with 4X. Though the choices are limited, they are still available. Leupold, IOR and Zeiss come immediately to mind, though I am sure there are others. I believe Zeiss no longer makes the 4X Conquest (I'm not sure of this). As someone else has stated, you can have a custom reticle installed in the Leupold and both the IOR 7A reticle and the Zeiss Z-Plex reticle work good in low light, though I've only heard this from others about the Zeiss, since I've not yet looked through one.
Good luck in your choice. Have fun with your 350 Mag. I have.