Ignition issues

[PowderB]

I recently bought a 1975 Gmc k1500 Sierra Grande for 400$. The kid said the lock cylinder was bad and already had a new one to replace. Well problem is since i put the new one in the thing seems backwards. I have replaced millions of these things but this is a first. What is going on is, when you roll the key forewards in order to start the thing, it cuts the ignition off. When you roll it back to kill the ignition normally and lock it the thing starts. What do i do to fix this and make it work like its supposed to.