I am wondering if your lower leg is not getting the magnesium you are taking orally.

If blood vessels are constricted, blood flow is less and things you take orally cannot get in there.

Magnesium comes in a cream:
http://www.kirkmanlabs.com/ViewProdu...ct_ID@124.aspx

It is designed to give 100mg mag in 1 gram measurer (which is provided).

I'd try this way to improve your circulation and this may help the leg cramps. If you still take your oral mag, cut back 50% and don't apply more than 200mg/day of this cream.
I'd apply it 1-2 hrs before going to bed.

If your cramping is totally due to nerve damage from your back, there is not much you can do to help that if the magnesium doesn't work. You need the magnesium for other metabolic functions so it is not wasted if it doesn't work on the leg cramping specifically.

There are amino acids for circulation, too. Since you are having chest pains, and using nitro, however, that would preclude using that approach. Nitro comes in long acting form... called Imdur. This lasts all day and may help more than just taking the little sublingual tablets when you feel the chest tightness. The Imdur may improve your leg circulation too. This you will have to discuss with your doctor.