727 jumping shift detents

I suspect the detent spring is worn or broken as it is quite a soft movement, just fingertip for about 1 or 2 inches of movement per detent.

I've been through all the linkage points and they are as good as they can be,
Are you saying that the lever on the transmission has 1 or 2 inches of movement before it clicks to the next detent? Cuz that would speak to a broken rooster comb..
But if yur talking about the shift lever on the column, then you got serious problems, that need to be sorted out. You can't race like that! I would start by looking at the bottom-of-the-column bearing. Followed by the various bushings and pin-holes, and ending at the top of the column under the steering wheel. Or even simpler, what's retaining the column lever?