Can you swap a GM 350 engine with 1964 Buick 300 without modifications?

No, you cannot swap a GM 350 engine into a 1964 Buick Skylark (assuming that's the 300 you're referring to) without modifications. While both are GM products, there are significant differences in the engine mounts, transmission bellhousing patterns, and potentially the oil pan, requiring modifications to the engine bay, transmission, or both. The accessory drive locations and configurations are also likely different.