too much small block ?
 

My brother is building a 20 foot open bow EBKO boat and bought a best friends 355 SBC out of his drag car. Well time and funds don't allow allot of changes to the motor yet and so he wants to put it in and go boating for the rest of the summer . But it has 12:1's and a cam with 578 lift and 260 degrees of duration . Some say that you have problems with reversion but also some say that is you extend the center outlet a few more inches towards the transom it will work .Any input there?

Does he have enough money for a new set of tailpipes? Most likely it will revert and then he will have to change back to the old engine or put dry tailpipes on it. Also 12 to 1 compression will require race gas. I wouldn't do it.

drag engines spike the rpms for a very short period of time....marine application will have that engine at high rpms for extended periods....probably will not last.....will most probably have reversion at idle and definitely will require race gas....not good idea

The torque curve of a drag motor is too short and peaks way too late to be of use in a boat. Marine engines should have as flat a torque curve as possible.

What are the Idle Characteristics of this engine?

I'm assuming stock manifolds are going to be used. With that size cam it's going to revert water horribly which is going to lead to an early end to the motor. I had to add 8" extensions to my risers to prevent reversion with my small 226/230 112LSA cam. Do it right and rebuild for a marine application.

Like everyone is telling you, swap the cam, and lower the compression...