You can pick up a conversation kit from Shift Works to convert the 2 speed shifter to a 3 speed.
I'm always confused by the statement at Shifteorks website that states the Powerglide shifter can be used without modification on a TH350 or TH400.
1968 400 Coupe, verdoro green, black vinyl top, auto 1969 GTO Judge coupe, Carousel Red, manual 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver), Fire Red, white top, auto 1972 Buick GS Stage 1, Royal Blue, black vinyl top, auto
