Couple of things: First, the factory tin head gaskets are only about .015 thick compressed. Your typical over the counter fel pro head gasket (like I used in the Summit kit) is about .050 thick compressed. So, as you can see, .040 is only going to truly net me only about .005. Now, at the other end of the head gasket spectrum, if you go with a .028 compressed performance gasket, you are still only netting about .030 from the factory.
- Trust me on this: Factory blocks have different deck heights and cylinder heads will vary as well. Not all engines were "true" from the factory and all used the same push rods from the factory be it 318,340 or 360. My point is, hydraulic lifters will generally be for given of a .030 gain (probably the best you'd do with a special thin gasket). Quick test is once assembled, with the valve shut and cam lobe on opposite side of lifter, your rocker should have slight "wiggle" room.
- I have never run into a problem with cutting the heads .040 and using stock push rods with hydraulic cams.
- Last, if you wanted to measure to be on the safe side, not a problem. Measure :)
