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.