In an earlier thread you mentioned windows in your garage as a factor you worried about. Did you get the shooting area free of ambient? I am guessing you did but just mention it to get a confirmation. You seem to understand that ambient light is unwanted, except for modeling lights.
I agree with others. Except in some editorial shoots, portrait success is helped by smaller apertures than the settings you seek. In the portrait you shared earlier a couple stood or sat facing you with shoulders overlapping. Even if you pose with shallow depth of field in mind you cannot keep both pairs of eyes in exactly (f2.8-level-exactly) the same distance to camera. Further, subjects move after your finger starts pushing the shutter release. It adds a risk factor that most portrait sessions and fee structures cannot support.
In your studio, you may have tough limits on how far behind and how wide your background can be, but I urge you to see how great a distance you can squeeze out of your floor plan. Find wide enough material for the tones you seek. This also offers the chance to incorporate lighting control over background through the use of another head. Fun and cool. Then the smaller apertures are not bringing background materials into sharp enough focus to cause issues.
A third note: ISO 50 isn't a good idea if the dPreview review of the 5Dmk3 is correct. They showed a truncated tone curve for that speed that was weird. Check it out and see what you think.
If you stuff more diffusion material into the softboxes watch out for color shifts.
The softboxes seem a bit low, although you described them as above the face level. I usually like more shadow. Not sharp, really but more tone range, so your goal may be different. Is the inside of your shooting space dark enough to insure that light bouncing around will not influence the image in either color, amount, or direction? I used to have to keep my walls dark grey or drape black cloth around the set in my studio space. I had more room, though, and higher ceilings (here I am guessing) than your space.
Keep up the experimentation. Vary the lights, move one back to make it a fill rather than shared key. Go with one light and a 4x8 white card right outside the frame as a reflector. When I started all this experimentation cost lots of cash (film and processing!) and was not rewarded with instant feedback except for Polaroids from the 2 1/4 and 4x5 (I did have a Pola back rig for the Canon F bodies that made a soft 1x1.5-inch image on pack film. Hard to see details! Anyone here remember that crazy thing? Hand made by a small Boston firm.) I betray my age.
Have you considered hanging them from rafters? Fun and keeps the floor clearer, but you have to work out a way to move them. Do-able if you are a Mcgyver type.
Go for it!