I've been a heavy cramper all my life. What works for me is SaltStick capsules, two taken every 20 miles or so (I have no BP issues). If I fall behind on my intake, they can even relieve my cramping after it starts.
They contain 4 electrolytes, the main one being sodium.
If they put enough sodium in sports drinks to work most effectively, people wouldn't want to drink the stuff. Capsules get the electrolytes into the system without concerns about the taste.
HTH.