You have an early engine and a later tranny. It is easier to go the other way, using a thin bushing than several here sell. One option I have read is to change both the torque converter and tranny input shaft (I haven't done so). If you still have the old transmission, consider it.
The OP's vehicle is an '80's truck w/a 727, there is no changing anything to make it a small hub, period. Nor would it be desirable, converting a late model A998/999 the same,
too many differences in front pump, support bushing, drum/clutch assy., plus You'd be undoing 20 years of improvements....................why? Cheaper & easier to get the right
engine or crank w/the appropriate post '67 hub register. Crap, I mean people are looking to give the damn things away to anybody willing to swing by & pick them up out there
in CA, just look through the for sale listings here!! Plus, someone out there is looking for the reverse for their pre '68 A-body, sell/give/exchange Your '67 mill.