stock rocker spacing.

I think what Rumble is saying(not to butt in) Is that your rockers should never be loose enough to slide back and forth easily. There should always be pressure on the rockers even at tdc.(that would be the preload). You may have some worn lifters, rockers,pushrods, or springs.....(Check the ball area of the rocker arm and the pushrods first)I have ran the stock rockers on a 318 and a 360. I always had a little clearance between the rockers, I don't think it's a big issue on street cars. Just make sure you have the rockers on right and the hold down spacers in the right places. For the most part if everything is on right and in good order I believe the rockers and valves pretty much self align. If you can move those rockers from side to side easily then you probably don't have enough preload and will most likely make for a noisy valve train. There are also places that sell the rocker arm shims to eliminate the play your talking about.

P.S. Mellings MPR-324 pushrods are 30 thousandths longer than stock and have the same ball/ball ends. I used these to add preload with good results.