I'm not sure why a later 400 would be needed. 65 and up had park except for the heavier trucks. But, you might drop the transmission pan and with a flash light look towards the back of the transmission to see if the slotted disc for the park pawl to engage is there. You should be able to see the slotted edge just barely. They may have simply left the actuator rod off so it could not be placed in park. You might get lucky and simply install the linkage inside to complete the parking pawl engaging the slots in the disc.

Or find another 1968 TH400 and install it. Check overall lengths and whether or not the driveshaft yoke slides in or is held on with a nut.

You might have to remove the tail cone to install a spring and part of the actuator, but that's not hard to do,.

One other option is to take the transmission down and take it to a rebuilder and they can get the parking mechanism set up.