Steering shaft diameter

3/4".
But since installing one of the "kits" will require you to remove the steering column and cut & shorten the shaft, consider placing an ad for a power steering column for your Dart. There are guys on this forum looking to do the opposite- go from power steering to manual- and would give their eyeteeth for the longer manual column. Given equal condition, a straight up trade could save you some bucks, especially if you're not doing the work yourself (and take the guesswork out of indexing a cut column to the new coupler/U-joint).