I checked the kickdown cable, and it seems to be adjusted properly. The car shifts really hard at low rpms, there is a hard clunk when it upshifts or downshifts. I'm planning on a rebuild soon anyway, but is there anything else that would cause this?

 

Is there any kind of valve body (shift kit) kit in it?

 

i don't know. It always behaved that way since i bought the car a year ago.

 

As Dana mentioned possibly a shift kit also could be a broken accumulator spring or stuck one. It would definatley be accumulator or valve body related