i am a dealer for airride tech. i will help you with your system , i have had airride since airride was not even a company !

you 64 will need a coolride system , with a ride pro e( thats the 4 way system as sick65 was talking about ) it is a easy system to use and install . i have used other companys and have NEVER been happy with the product .
as for the 20 and 18 , i also am a dealer for many brands , you can run 20x8 and 18x7 without any problems . most wheel companys will tell you want backspacing to use with stock or lowered suspension .
hope this helps you