I think Steve's touched on most of the relevant points. I'm not much taller than you and during my time with the M14, I did use a magazine hold. I still use a magazine hold with the AR Service Rifle.
I think getting your left arm supported by your torso is worthwhile enough to offset the slight theoretical unsteadiness brought on by a rifle balanced above a magazine. Folk with longer arms are able to use a left hand hold on the Delta Ring om Standing. I can't without pointing into the dirt. Experiment with different positions, but remember bone on bone up high trumps muscling down low.
Sitting is the same for me. I pull my left had backward probably 2 inches forward of the Delta Ring to give me the elevation I need to get on target. Without being able to see your position it'll be just a guess, but I'd be you're trying to shoot Cross Legged and your elbows resting on your knees don't let your head get down low enough to cheek wood? (been there, done that) Try sitting with your legs laid out in front of you with the ankles crossed. When you lean forward, I'm guessing your elbows will get out onto your shins, but your face can reach the stock now? Just an educated guess. You might also find it more comfortable and stable to "cant" the rifle to the right. Remember...stable is good...
For Rapid Fire with the '14, remember to sling up as tight as you can stand it for 40 seconds and get your weight behind the rifle. Pull the rifle back hard into your shoulder WITH THE RIGHT HAND ONLY for added recoil control.
Have fun.