In the very different world of class racing, Stock and Super Stock in particular, very stringent rules regarding what was allowable and what was illegal have been in place practically since this sport's inception. Some heads just seem to work better than others, after all, they are mass produced castings, so variations are an inevitable occurrence. Fudging on the rules resulted in NHRA issuing a set of specs for intake and exhaust ports as to the number of CC's the port could hold. Practically no head right out of the box is any where near the maximum port volumes so some racers get a bunch of heads of a given alllowed number for the combination they are running, and they found quite a bit of variance. The best two heads were set aside for the car's best motor, the heads then were rated as to how they flowed, whereby the racer would keep the best few sets and would sell off the rest. One thing that can cause variations in port volume is what valves are used.Some tend to be a bit thinner than others, they might have a different profile than others, even though they are all perfectly allowable valves for that application. It's a time consuming process matching and mixing parts to get the desired results. One thing that used to happen a lot was acid porting the heads, using acid to etch the ports oversize. This was hard to detect, but a trained eye could spot it. If the heads were a bit oversize and were checked, they would hold more cc's of fluid than the spec. One way this was compensated for was to surface the intake gasket side of the head. That was a good way to lose excess port volume, but the logical extreme resulted in compromising the valve cover bolt holes.NHRA then came out with a rule amendment stating that no portion of the valve cover bolt holes could be penetrating the intake gasket surface. How about if you race a Ford FE motor? Also, it can take up to .050" to get a block's deck height down to the minimum spec for deck height, and as much as .080" or so off the deck surfaces of the heads in order to get the combustion chamber volume where it belongs. After all this gets done and all the specs check out, the push rods can be up to .125" too long. Custom length push rods from the aftermarket are the answers here. Push rod length can be pretty critical here, here's one more thing to consider. Getting the engine up to a long block now means that the intake manifold is too wide.Finding a machine shop that can face off the front, rear, and side intake gasket surfaces is essential here. As you can see, building an optimal motor is quite a job. Another thing that can bite you is what happens when the distributor bottoms out into the oil pump drive shaft before it seats into the mount on the intake manifold. Decking all this stuff makes the total height of the motor shorter so here's another one for the books. Somebody makes a distributor spacer to take up some of this excess length. Does it ever end?