Milling the heads would increase preload, not decrease it.
The process for checking valve lifter preload:
1) dissable the fuel pump and ignition system so the motor can't start.
2) turn the motor so that one pair of valves are closed. This is the "back side" of the cam. Back off this pair of rocker arms so that they have at least .060" clearance to the valve.
3) crank the motor over 2 complete rotations. This pumps up the lifters. Check to make sure you still have clearance. If you don't, back them off another .060" and crank the motor again. Both rockers should now be loose. The exact amount doesn't matter, just that after cranking they are both still loose with a gap to the valve tip. I don't like backing them off more than .060" at a time because the pedestals can turn and the bosses are easily damaged.
4) now that we are sure that the lifters are fully pumped up, and the cylinder in question is on the back side of the cam (both valves closed) you can tighten the rocker bolt with your fingers. Spin it down until you hit definite resistance. This is .000" lash.
5) Now put a wrench on it and noting the degrees of rotation required, tighten the bolt until it stops. Torgue is not critical here, just tighten it until you feel a definite "stop". Note the degrees of rotation. It should be between 3/4 turn and 1 1/4 turns. Marginal would be 1/2 turn to 1 1/2 turn.
Final torque values do not affect lilfter preload so don't worry about that. This is done totally by feel.
If you find that your rockers tighten down within the prescribed range, the it will be necessary to back off all rockers using the above process and check all valves because evidently you have one or two that are out of spec.
However, if you find that preload is low (1/2 turn or less) you will need longer pushrods. Check your length and order the next size up as well as some rocker shims (you'll need both).
If the preload is already high (which I doubt being that you have noise) then you will need shims already.
You may be experiencing normal valvetrain noise. Roller rockers are noisier than stock, and high lift cams will cause a certain amount of noise all on their own also. It most likely won't ever be totally quiet.
If you do all of this and still have unacceptable noise, the final option is to replace the lifters as you may have one that is sticking. Make sure that they are installed with the oil orfice facing outward towards the oil galley.
{edit} Steig heads ARE bolt on in as much as they can be. Bob retains the stock valve tip installed height. However, as with any high performance application, nothing else is stock dimension so in final assembly you must check your clearances. Even SBC and SBF people must go through this step.
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