"I have a book from Nissan Motorsports that lists the chamber volumes as:
E31 42.4cc
Early E88 44.7cc
Late E88 47.8cc"

Actually there were 3 E88 heads. The first casting still had the good quench kidney bean like E31 combustion chamber, but with the E88 port layout, which flowed slightly better than an E31. This is the most desireable head for a 240Z in ITS. Call it the early, early E88. The early E88 mentioned above had a boss around the spark plug with what one would call the normal E88, and later N42, shaped combustion chamber. The late E88, which showed up in '73 (with the tall deck, valve relief pistons), had no boss around the sparky plug.

The general rule of thumb for reducing the combustion chamber volume on an E31 or E88 head is about 1cc reduction for every .010 you shave off. However, that's very general. For example, one of our ARRC E31 heads measured 41.1 cc at a thickness of 4.235 inch, which is 4.248-4.235=.013 shaved. You can get an E31 or an early, early E88 down to 37-39cc even when staying within the allowed .025 head shaving rule. That puts you way over 9.5:1 compression ratio, the other limit. Crushed head gasket volume is a huge player, and even the factory gasket varies between 7.2 and 7.8cc so you have to be really, really careful about how much you shave the heads to stay under 9.5. We eventually had our own design solid copper gaskets made so I could control that variable.

"It seems to me that in sum, with a Z car, there are a few keys to speed:

1. Use throttle as an on/off switch.

2. Get car to handle, especially with rear grip, so that you can do (1).

3. For power, the key is timing and carb (especially piston dampening) work on a dyno.

On base? Off base?"

On base. I'd add having enough money to buy and manage your race tires. Lock the timing down at 34 degrees and fuggetaboutit. Properly instrumented tuning (carb) is where its at. Of course, some low tension rings, precison head work, perfectly balanced internals, proper headers, early 71 flywheel, 4 post plugs with Jacobs ignition, blah blah blah, and letting the ozone out of the distributor cap all matters too. i.e. the WHOLE program. But stomping on the gas before the other guy is most of it.