How to get 400 hp out of 1971 340 .30 over

To get 400 HP out of a 347" (340 +.030") requires that it makes 1.153 HP per cubic inch. However if you had 416" engine you only need to make .96 HP per cubic inch to get 400 HP.

That said it all boils down to the amount of CFM your heads flow as to what kind of HP you can extract out of a given cubic inch.

Tom