Granny car parts manual is what you want, gives the part number, item description and number used. The part number has all the info on the fasteners, you just need the code to decipher them.
This will help, http://datsun1200.com/modules/mediawiki/index.php?title=NES_Part_Numbers#1-_Bolts Applies to the 1200 but pretty much the same for all Nissan vehicles. Worls for the 240Z, I used it to sort out all the fasteners for my cars.
Here's an example:
Example: 240Z machine screw for the crankcase mesh filter cover, number 25 in the diagram http://www.carpartsmanual.com/datsun/Z-1969-1978/engine-280z/cylinder-block
08310-61214
08=standardize hardware prefix
31=Machine screw, Pan head
06=6mm diameter (M6)
12=12mm length
1= Zinc cadmium coated
4= filler to complete ten digits