NOTHING will be a bolt-in swap. Although Oldsmobile & Pontiac used the same 6 bolt top cover selector type trans. Yours will have to be removed & rebuilt/repaired.
Dennis at DME in Kentucky can rebuild it. He may have the NOS parts. DME (Dennis Maroudas) @ 606 237-6648 =Forest Hills KY
The difference between the Olds/Pontiac transmission and the Buick box is the torque tube. You could use an Olds or Pontiac transmission, but the extension housing and output shaft will have to come from the Buick transmission. BTW: That transmission is quite rugged. Like the Olds and Pontiac "selector" boxes, it will not tolerate speed shifting. With normal shifting, those boxes will normally last pretty much forever.
Hey guys, what front end can I put under this Buick? Hopefully one that I can get parts far, I am new at this and I am not a mechanic