Trunk Lock Extender Bar vs. Key Lock Mechanism

Hi,

I am going to replace the Trunk Lock parts on my 66 Mustang Fastback.  From all the Internet articles and discussions I have read, I understand how to remove and install the trunk latch, extender bar/stem, cylinder, and key lock mechanism with the "push-pin" down.  I have purchased all the new parts and am ready to install them.  However, after "practice installing" the new parts, I have one question given the following explanation.  

The new extender bar/stem goes into the new lock cylinder with the larger "female-slotted end" of the extender bar/stem facing upward or going in the cylinder last.  Next, the "key-in" locking mechanism, with the rubber o-ring and lock pin down, goes all the way into the lock cylinder with its "male-slotted end" mating up with the "female-slotted end" of the extender bar/stem.  Finally, you move the key a little, the lock pin comes up in the lock cylinder slot and you remove the key from the key mechanism.  However, I have noticed there can be enough movement to allow space between the "male-slotted end" of the key mechanism and the "female-slotted end" of the extender bar/stem preventing the trunk lock from working once the smaller end of the extender bar/stem is installed into the new trunk latch.  What am I missing that prevents the "mating-ends" of the key mechanism and extender bar/stem from coming apart during the Trunk Lock installation?

Thank you,

Allen