Is it possible to change my 4-speed automatic to a 6-speed automatic
 

i have an 83 berlinetta, 305 v8, 4-speed a/t. is it possible to change it to a 6-speed a/t?

I recall reading an atrical about GM just making its first 6spd auto. Im sure you could swap it, but I bet the thing is $$$$$$$$$$$$$$$$$$$

Why and what are you trying to achieve? If you are going track, use a manual T5 because I doubt you will use all 6 speeds. If you are going down the quarter, use an auto. You want to make sure that it gets you in the right RPM range

Only if you install one of these:

http://www.gearvendors.com/