As stated above, the rocker arm to valve stem head contact point will not change no matter how much you mill the head if the correct push rods are used. If you do nothing but bolt the head on using stock push rods, then yes, you have changed the rocker contact point and will wear the valve guides out much quicker. You can not shim the whole rocker stand higher to restore this. The rockers must hit the valve stem head at the correct points all across the whole opening and closing values.
There is only one good way to fix this, get shorter adjustable push rods.
Recessing the valve in the head will change the contact points a little, but I would think you would would really have to sink it deep to really need some rocker shaft shimming. If this is the case, you might think about new valve seats to restore the factory valve stem hight measurements. If they are not to bad, the adjustable push rods should compensate for the machining.
On a lot heads, its not a one to one process for milling the heads and raising the compression. If the chamber is tapered, more compression will be added the deeper you cut. So the first pass may only raise the compression a little if any, the second pass will be more and it will multiply as you go. So its pretty hard to come up a number for how much cutting will raise the compression. You need to find a stock head and measure each to get an idea, and then its not 100% since each head is slightly different and with there age, the history becomes sketchy.
Mock up the head on the block and get an idea of the push rod length needed to place the rocker arm in the center of the valve stem at about 2/3 lift on the cam. With this measurement you have a starting place to work from. Milling 1/8" off the head should mean a 1/8" shorter push rod if all was correct before the machining.
Joe
