Will a th400 fit into Chevy 6.2 diseal?

No, a TH400 transmission will not directly fit a Chevy 6.2L Duramax diesel engine. The TH400 is designed for gasoline engines and doesn't have the necessary adaptations for the Duramax's torque and output characteristics. The Duramax requires a much more robust transmission, such as an Allison automatic or a suitable manual transmission designed for high torque applications. A direct bolt-on is impossible; extensive and likely impractical modifications would be needed.