|Since this has been coming up lately, here's a practical solution to all the related questions about either pushrod length or lifter preload.
In General, the proper lifter preload for a LC2 motor is .030 - .035 inches. More preload will give you a stronger top end and less preload, a stronger lower-mid range. One easy way to measure this is to A) remove intake manifold, B) rotate engine to TDC on any one cylinder, and C) using a wire-type spark plug gapper, measure how far the plunger of the lifter has been pushed down from its seat (thanks to John Pearcy for this one.)
On a motor where you have changed something in the valvetrain (camshaft/lifters, head thickness, new/different head gaskets, new rockers or roller rockers), you need to determine the proper length of pushrod to attain the amount of lifter preload you want. To do so requires that you buy some .010" shim stock (available at machine shops, larger NAPA's, etc.) to make some shims (cut out a rectangle and drill hole in middle.) Using the pushrods you have now, install the rockershafts with X number shims between the rockershafts and the mounting bosses on the heads. Torque the shaft bolts to 25 ft/lbs. X will be the number of shims required to attain zero lash. This is attained when the rocker does not move around on its own and there is zero preload on the lifter. If, for example, this occurs with 5 shims (.050") installed and you want lifter preload to be .030", then the proper pushrod length will be .020" shorter than the pushrods you are using presently.
Note most (all?) Roller Rockers are adjustable so the above procedure can give you the rough estimate and you can adjust from there. Yes, you can run the motor with the shim stock installed, but it isn't the wisest thing to do.