You can go with a transgo shift kit that will let you manualy shift it as well as work as a normal automatic.
http://www.transgoperformance.com/gm700r4.htm
GM 700R4        Kit: 700R4 PSK  Automatic Shifts
       4 Speed Rear Drive                      
Stick Shift Option Included
 
1982 to 1993:  2.8, 4.3, 5.0, 5.7 Cars, Trucks, and Vans.
 
Features:  Street Show-off to Full Race:  Perfect for Transplants. Manually hold 1st-2nd-3rd to any RPM.  Manually downshift to any gear at any speed with the shift lever.  OK to 600 HP.            
   Stick Shift:  All shifts are made by moving selector. No shifts are automatic.