Isky sets a valve notching tool that is shaped like a valve, it has a pilot that passes through the valve guide thereby matching the angle of the cutter to the angle the valve would contact the piston...Basically you asemble the short block, bring the piston you'll be working on to TDC & put the head on with the cutter in place of the valve..Cut the relief then move to the next valve & then the next piston...
Go to the Isky site catalog, look for tools, then notching cutters...