Needing help

[LetsGo]

Hello all
I need some help with a charging problem battery will not stay charged. Let me start from the beginning I had an 85 mustang with a 351w carbureted that I took to Germany. Someone decided to hit it and that brings me to the 91 mustang that I have now. While searching in the newspaper I ran across a woman selling her mustang do to fire damage caused by the alternator. I purchased the mustang and started taking it apart. I had no problem with my 85 charging system but once I put everything in the 91 that is when the problem started. I have change the alternator to an 85 amp along with the wires coming from the alternator to the harness connection. I can ride the car around for a while but when I turn it off it is hard to start up again it will turn over but want start. When letting it sit for a while, it will starts back up but the next morning the battery is dead. Any help that could point me in the right direction would be greatly appreciated. Thanks for all your help and ideals in advance.