i have said this before in another post that people here didnt want to believe but if you have your block bore and honed with a torque plate you will see a big hp gain for doing this,and the other thing that might be better to do instead of milling your head down is to zero deck your block,mock up your new pistons in the bore and see how much they are in the hole, if they are .020 down mill the block .020,then you would have a zero deck block,much better for the burn, you can mill the head some too if need be.
Zero decking increases compression in the same way milling the head does but valve train geometry remains undisturbed so I have to agree that's the best place to start. Torque plate boring is also a magic ingredient IMO but the scarcity/expense of them is what likely would make a rebuilder downplay the benefit. It's a good idea to get in the ports with a die grinder and knock down the barnacles/casting flash but major alteration or resizing will have little benefit. Flow is mostly generated by the area close to the valve seat so a good 3 angle valve job will give the most bang per dollar of flow improvement.
