Ok, I pulled my shifter out to install the detent conversion kit for the overdrive transmission and I'm currious if my shifter is missing a part because the slider does not engage all the way the the cuts in the detent bracket.  Let me try and describe.

There is the shift tube that you see in the car.  On top the chrome tumb button screws on to an internal slider.  On the outer tube is the grip knob that screws on.  When you depress the chrome thumb button the internal slider releases from the lower detents in the bracket so you can shift from park to reverse etc.  At the highest point on the detent bracket is neutral and drive.

The internal slider has a flag on it rides inside the outer tube.  The flag of the slider is suppose to come all the way up and engage into each slot to prevent the shifter from hap hazzardly going between gears.  The problem is the slider does not go far enough up the slot to engauge the neutral and drive postion.  This means you can just pull back and get 3nd or 1st at any time.

I can manuall get the slider to engage the detent by pulling up on the internal slider.  I took the slider out and there is a little washer on the end of it that actually rests against a step in the outer tube so stretching the spring that fits between the slider and the lower threaded spring seat has no effect.  It appears that I can correct the slider to detent engagement by placing a spring inside the grip ball under the chrome thumb buttom....... or is there suppose to be one there and its justs gone missing?  Even if there is not suppost to be one I may add one.

Thoughts?

There should be a spring to hold the "internal slider" w/'flag' in the 'up' position, therefore engaging with the detents on the shifter frame, for each gear position. When you push down on the ball button, it would release the 'flag', and allow movement of the shifter handle. It sounds like the 'flag' portion is not staying in the 'up' position. Is this correct? I can't viualize the setup right now, but I believe there is a spring under the button, and the button screws onto the rod that attatches to the 'flag'. This spring would hold the button, and the rod w/'flag' in the upward position. If it ain't there, and putting one in will cure the ailment, go fer it! It appears that the one spring that is there has lost some of it's 'spring'. Put one under the button, and see hows it work?!

looking at my 53-72 GM parts book it shows 65-67 PG linkage as follows, on the bottom of rod assy down by the flag it shows a plug, spring, and a washer, which then slides into the shifter tube, looks like the plug screws into the shifter tube and the spring holds the flag in the up pos.,when I go to 68 turbo-hydro linkage its just our luck it doesn,t show the shifter tube detail like it does for the PG,but looking at both the outward views of both the 67 and the 68 shifter tube assy's they look the same.when I go to the"gearshift control(hand) lever" parts list, it shows a control cable lever, cable retainer,control lever pin, control lever plug, control lever upper spring for 68-72, THIS might be important,when I go to the " lever-button-knob-rod"gearshift control there is a note" In some cases it may be necessary to cut or grind off .0001 from the knob end of the lever to allow travel of the botton that sets inside the knob to shift into all positions". anips38517.8268055556

What you described is correct.  The problem is that little washer on the end of slider actually prevents the slider from coming all the way up inside the outer shifter due to a seat/lip on the inside the outer tube.

But I figured it out.  I was looking through my CC catalog and found a parts diagram of the automatic shifter.  Turns out that there is a spring that goes inside the knob under the chrome thumb button.  Makes sense because you would want the lighter spring on the detent to shift between drive, 2nd and 1sft and the heavy spring to shift it out of park and reverse.  I'm going to order the spring.  Expensive spring for the purpose it serves, but at least it will have the right tension and fit.