[GRX]

x2 on the 400 SBC being external balanced. Was going to mention that until I saw burnin oil's post. Have seen a lot of guys go wrong thinking they can change flywheel/flexplates at will like other SBC.

Purely as an aside .... an interesting study is how the Chevy and Pontiac 400 share the same stroke and bore size, yet have very different torque profiles. Largely due to different head design and subsequent flow characteristics.