I just purchased a used 351 modified. I stripped the motor down to the block, cleaned it up, and put it back together. When it came time to tighten down the rocker arms, they were real sloppy against the push rods, and valves. My first thought was that the lifters were bad, so I replaced them, now they are even worse than what they were originally. I have had 4 or 5 of my back yard mechanic friends look at it and we are all stumped please help