Welcome to the site. Have been watching your discussion over on the SAAC forum. Lots more of those cars built with comp suspension in 67 than Mustangs with the same package and an automatic transmission and would guess of that handful many never looked at the shifter modification. I know I never did back decades ago when I had one.
Can't recall if I've posted these following pictures on this site or not. Some are mine and others have been shared by others.
Unfortunately I didn't try and make patterns for both the upper and lower modifications when I had the opportunity
A1- Small lower plated that the indent shaft connects to. The adjustable shaft has a inner moving detent shaft rounded at one end and is spring loaded
A2- Lower indent shaft
A3- An example of the spring loaded end that road across the plate added to the base of the shifter.Please ignore the shinny nuts they used on the shaft to remove it from the lower shift arm in the upper left of the picture
A4- An over all look from below
A5- Upper detent plate added/modified