I converted my 68 Cutlass to a 4 speed... The mechanics of doing the job are fairly simple, assuming you can find a donor car with all the right parts.
Pedals - drop out, install new ones.
Bell Housing - assuming you have a BOP engine, Oldsmobile bell housings are plentiful.
Crank Shaft - You would need to make sure that the engine parts (crank, balancers ect) you are using are compatible with the flywheel.
Shifter - Go with a Quality Hurst shifter, on the floor. you only need a 4 inch or so hole in the floor. Fold the edges over to maintain your strength.
YOu might even want to find an Olds 350 with a manual tranny set up and swap in the whole moter ect... save the Buick engine for when she gets tired of shifting.
Transmission - Ensure that you get a "WIDE RATIO" tranny.. unless she likes to shift 3 times before she hits 30 mph.
I converted mine back after a couple of years, the 4 speed was fun for a while, but got old.
