Yes, I've done it. But the number of 5-round boxes a .50 can will hold varies with the overall length of the shell- even so-called 2.75" shells can vary in OAL depending on crimp type etc. Only way to tell is to start stackin' and fill 'er up. Even if the can won't hold as many, I'd rather store the 5-round boxes as units rather than as loose ammo.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.