I've done the swap many times... The guys covered much of what you need, but for my own sanity.... I'll cover the entire list:
Trans - from a '65+ Pontiac, flexplate
AND BOLTS, torque converter, starter (use for a core on new one), linkage from frame to tranny, driveshaft (for yoke), kickdown switch and wiring/mounting brackets.
Once the trans is in, and you have the extension plate made for the trans mount (I have the pattern if you need it)... measure from the end of the tailshaft housing to the end of the U-joint connector on the rear end - this is the measurement your driveshaft maker-guy will need. Make sure the car is level and on all 4 wheels to get this distance (or make sure that the rear axle is on jack stands vs the frame on jack stands so that the "load" of the car is on the suspension). - All he'll need is the yoke from the TH400 driveshaft. It should run you about ~$125.00
You will need to modify the shift linkage to adapt to the TH400 with a combination of the old frame to trans linkage and the new TH400 frame to trans linkage - some welding will be required.
Since you're in a Star Chief - the trans tunnel will be large enough to accept the TH400 without any floor pan modifications (i.e. use of a BFH not required)
Take the old starter, use it for a core, and get a new one.
You'll need to locate a vacuum port for the modulator on the trans - off of the back of the carb (a "T" works well).... if there is hard tubing for this on the donar car - I grab it also.
Have the trans rebuilt
With a 1/2" knock-out punch, you can place the dipstick tube fitting from your Super Hydro into the pan of the TH400 - takes about 1/2 hour and some silver solder to put the fitting into the new pan - send me both pans, and I'll do it for shipping.
The rest is pretty much easy - drive and enjoy your Pontiac
Ron