Pistons: All L-series engines come with flattop pistons. All except the L28 in the '75-'80 280Z. They used dished piston to lower emissions. Dished pistons don't allow for as good of a combustion, thus reducing power. If you pick up an N42 block, make to pick up some new flattop pistons as well.
Blocks
E31: Block on L-24 engines for 1970 only.
P30: Block on L-24 and L-26 engines from 1971-1974.
N42: Block on L-28 engines from 1975-1978.
F54: Block on L-28 engines from 1979-1983. Known as the "turbo" block.
Heads
E31: Rare cylinder head used on very early 1969-70 240's. Only about 10,000 made. Highly desirable- worth it's weight in gold. Used high-quench combustion chambers but the valves are actually smaller than those on the E88.
E88: Common cylinder head on used on all carberated L-series from 1971-1974. Came in 2 versions- small valve (240Z) and large valve (260Z). Chambers are the came size as the N42/47 heads.
N42: Head used on the 280Z until 1977. Favored for it's square exhaust ports and suggested good flow. A popular mod is to bolt on an N42 head on a flattop L28 F54 motor to put compression into the 10's.
N47: Restrictive design introduced in 1977 and used until 1981. Had diamond shaped valves and inhibited air flow. First head to use steel seats for unleaded gas.
P79: Used on 1981-'83 flattop L28 motors. Uses an internally oiled cam, and has 53cc combustion chambers. A good cylinder head.
P90: The "turbo" head. Has an extra hole for coolant and the same sized valves as the N42. Also has square exhaust ports. Said to be the best flowing and popular among racers.
P90a: Another turbo head that used hydralic valve lifters instead of standard rocker arms. This makes the engine much quiter. Identical chambers and ports as the P90 and is said to flow the same. No real benifits other than quieter opperation over the P90. Replacement hydralic lifters are no longer made for this head and using it would be very expensive and not really worth it.
Datsun performance gurus argue that either the N42 or the P90 are the best cylinder heads. But really, all are good for any high performane street application except the N47. In racing, the E31 and N42 heads are said to be the best, and without a doubt, the E31 head is the best for carberation. If you have a turbo, you're stuck with a P90 or P90a, but I wouldn't mind being "stuck" with one. In realitly, all of these heads are good and you'll probably be happy with which ever one you choose.