Tokico Illumina (the adjustable version), position 1 and 2 suitable for normal road use. 1 is slightly stiffer than stock, 2 is my favoured setting for everyday use. 3 is a tough road setting for relatively smooth roads and 4 / 5 for track use. I have them on my 280ZX (yes, I can just about remember driving it), but the rears are no longer available for 280. The 300ZX Z31 version is only available for rear on a facelift model.
Tokico HP work great on just about any Z model. They are a gas shock / damper with a rate a bit stiffer than stock. I've used them on my Z32 SWB (available for sale BTW, in case you hadn't noticed) and fitted them to numerous customer cars. They work well on the 280ZX and help with the infamous rear end droop.
For everyday use and a nice ride on early Z models, the KYB gas cartridges are an economic alternative. OEM cartridges in Euro spec no longer availabe for 240Z, so your choice is limited there!