Unless they are going to offer an iron clad warranty on that motor, I'd run away from this "Deal" as quick as your little feet can carry you! There is no way a stock Volvo GM 5.7 small block with cast pistons and crank and rods will take 6 pounds of boost in a boat and stay together for any length of time. Unless that engine is purpose built with all forged internals and set up for turbocharging, that motor will be like playing russian roulette with three or four bullets in the magazine! Just my professional opinion.
Ray @ Raylar