[Safari Larry]

I have done this -- 1959 tri-power engine in 1955 stick shift 2-dr sedan. It bolts in and works GREAT (loved that car). Charlie gave good advice on the transmission durability. After installing the 59 engine and hotrodding it, I had the transmission out every few weeks (speed shifts didn't help!). But with experience, I could overhaul the transmission within an hour and have it back on the road. With experience, I got to where I could get the needed parts before pulling the transmission. That was back in early '60s when used transmission parts (Pontiac and Buick) weren't difficult to find and cheap. Different story today. As Charlie says, if you don't hot rod it, the 55 transmission should hold up.

1956-57 standard 3-speed is much stronger and will bolt in with same year bell housing. Use a 1958-60 bell housing to install a 4-speed or 5-speed.

Another issue to consider is the exhaust manifolds. The 55 exhaust manifolds will bolt up to the 59 engine and work OK but the 55 manifold ports are smaller than ports in heads. To get matching ports, use 57 or 58 manifold on driver's side and 57-60 manifold on passenger's side. You'll want to replace the exhaust pipes, header pipe is larger on the 57 and later exhaust. For dual exhausts, you'll need to cut a hole in the frame to route the driver's side exhaust pipe.