Can a 455 fit in a 66 cutlass conv?

 

Yes, with some work.
What is in there now?

 

Yes. A 66 442, which is essentially the same car, came with a BBO 400 engine.

 

Actually, very little work if the car currently has a 330 V8. As noted, the 455 is externally identical to the 400 that was available in the 1966 442. The 455 bolts to the existing motor mounts and trans. You will need a flexplate that matches the 455, as Olds changed the crank flange bolt pattern for the 1968-1990 engines. Use the W/Z exhaust manifolds from a 1969-1972 442. Get a radiator for a 1966 442, which will be larger than your 330 radiator. That's about it.