What /6 will fit

the 67 and earlier cranks have a smaller hole for the front hub of the converter and the trans has a smaller spline count...

the 68 and later has a bigger hub hole (i think it was around 2") and the converter splines are 27...

so for stock usage 67 and older with 67 and older, 68 and newer, 68 yadda yadda...