Alternator or battery???

So I've been having this problem where my car won't start. Background: 74 Swinger, 390 stroker, MSD 6AL IGNITION, new mini starter and new 70 amp alternator. Battery doesn't seem to like to hold a charge. And when it doesn't have a full charge it won't even turn over. Power is getting to the MSD and also to the starter relay. When key is in ON position the gauge shows just negative. It also is starting very slow. It does spark when I have full battery. When I drive and play the stereo, it shows charging but will fail to start again or crank very slowly if I stop into the store. Any ideas?