74mm is simply the second oversize for this engine. I believe that there is a third oversize of 74.5mm

If you want to use this size, then check the Repco catalogue. Datsun 1000 pistons used a flat top with a longer skirt than 1200's
The 1200 had a shorter skirt to clear the longer stroke crank when the piston was at BDC, & used a dished crown.

Repco, bless em, made both 1000 & 1200 replacement pistons on the same shorter 1200 blank. They machined a dish for 1200's & made em flat for 1000's.
It doesn't take Einstein to figure that what we have is 1200 flat tops here & since the price was the same, a little extra compression could be had for free.

This is what i used in the 1200 engine that i built for my wife's '68 Datsun 1000 two door back in about '83. No tyre smoker, but a nice little grocery getter.