I got bit one time by installing the crank gear for the timing chain in the wrong position, the engine ran but was pretty week, once it got over 3000 rpm, the rhoades lifters pumped up and the valves started tickling the pistons or "clattering". To check it, roll the engine until the intake or exhaust valve is completely open. Shine a light down where you can see the lifter plunger top out and see how much slack you have left before the valve contacts the piston
If your engine will rev above 3-3500 without losing any more power this is not your problem.