1965 727 won't shift into 3rd

Depending on the filter extension type they have been known to allow air in around them, and this could also cause your symptoms.
The solid billet type seem to have a lot less problems.
Got it back today and tried to drive it, after latest mods it's nearly undrivable. I have park, d1, d2 manually and when i did get it to shift into d3 seemed like it was going into neutral and lost all forward gears till i slowed and could get it back into d1. After it warmed up didn't want to manual shift into d2. He removed the spring from one of the servo's added a spacer so it was flush and didn't move, same as another transmission he had which was being used for drag racing. Said it would help shift tighter from 2cd to 3rd but seems it made it manual. The entire transmission is junk now except for lot driving... live and learn.