Electric ?

[Harlem]

I have a 71 gmc c2500 2wd 383 stroker, blah, blah, blah. AnywayI bought it and It was only puting out 10 to11.5 volts. So I got aload boss 135amp HO alternator, same problem. Well as I was cleaning it up and straightening wires and what not I found a loom with a connector the is the same shape as the one in the back of the alternator( all that was hooked up was one wire from the battery+. So I hooked the wires up to the alternator and wow what do you know 14.5 volts..... For a while then It would go to near 18volts to 12 volts then 10. Then at idle it wold go to 14.5 but as soon as you start to go again it goes back to the 10 to 12 volts. Now Im no electric genious at all. So can someone tell me whats wrong? I can try to answer any additional ?'s you have if you want. I just want to be able to run the electric fuel pump fans headlights and all the fun jazz without the voltage dropping to where the truck runs like crap and if I floor it , it dies. Help please.
Thanks
-Harlem