I put a Hurst dual-gate in my 68 Firebird when I went from ST200 to TH350:
Dual gate shifter conversion pics
My shifter is actually out of an Olds 442. I originally tried a GTO dual gate shifter but it was too "tall" for my tastes - stuck out of the console a little. GTOKen was successful with this swap by dropping the shifter down into the tranny hump a little.
The 442 shifter was a near "bolt-in". It has a form factor identical o the existing first-gen shifters. I really like the mod, and would reecommend it. Hardest part is finding a 442 shifter!
Regards,
Bob Trimpe