I did this about 20 years ago as a pup, with the trans in the car, and I suppose it took about 2 hours maybe. Pretty interesting job actually, with lots of tiny parts which are easy to lose. Just have a clean work area and you'll be fine.
IIRC, B&M had good directions (I installed a Transpack in my street racer back in the 70's)...
Timewise, for a newbie to trans work, all said and done I give the job a solid morning. Have a clean, well-lit place to work on the valve body and some carb cleaner/brake cleaner and compressed air to clean parts...
Since the trans is out of the car, it'll be easy to check other parts once the valve body is out, along with adjusting the bands...consult your service manual for further details...
I haven't done a C4, but I installed a B&M Transpak in the AOD in my '66. The directions were very clear, but ... you must follow them to the letter /forums/images/icons/wink.gif.
The most common mistake (and, yes, I made it) was not engaging the the manual shift lever propperly when putting the valve body back in. If you have no reverse when done, and "wacko" forward gears, it's most likely the cause.
I agree with the other posts, but would like to add the following:
1.) Be sure to check a manual for the torque specs and any sequence required to reinstall the valve body. You do not want to warp it. (These directions may be in the kit. It has been awhile since I have done it)
2.) Add an external tranny cooler. Heat and dirt are the two killers of automatic transmissions.
3.) You will love the shift kit and it will add to the longevity of your transmission. Firmer shifts = less slipping = less heat = less wear.