I stumbled on to this. It's not a factory part, but it may get you closer to some kind of answer. I know it's a 4 speed car so this probably won't do much of anything for you. Just trying to throw stuff out there.
https://sportcompactwarehouse.com/b-m-racing-70209-speedo-cable-generator-for-gm-converter-lock-up-control-part-70244-generator-does-not-come-with-extende?Year=1979&Make=Pontiac&Model=Firebird&srsltid=AfmBOoqf-50UOQJyp-0lddATmmtqGLpdFPZJth4t4QT5LaGD3PRT7U_nVEo