Go to the salvage yard and get the necessary wire from any full size Buick, Olds or Pontiac since almost all of them came with longshaft TH400s. All 1968 A-bodies have the TH400 downshift switch and gas pedal regardless if they have the TH400. These are typically not in the salvage yards. The TH400 dipstick and torque converter plate are different than a TH350. 7. If your car has buckets and console, the shift cable may or may not be correct for the TH400.
Some cables have an adjustment at the end and some are fixed.
Another item that you will need is the TH400 shift cable bracket from a Grand Prix with buckets and console. The bracket attaches to the trans oil pan and has different hole spacing for a TH400 than a TH350. Chevy rear is also nice to put in at the same time since these are quite a bit stronger than Buick 10 bolt rears. So do not overlook any A-bodies since you never know what you may find underneath. If you do find one and decide to put it in your car, you will have to change the driveshaft rear U-joint.