The "standard" SU's used for the datsun GX engine were HS4 size (1.5" bore). There were no HS2 sized SU type carbs used on any Datsun's that I know of. The datsun Z cars use HS6 size carbs (1.75").

So basically, the HS4 size carbs would be perfect for a 1.5L. I've posted this chart before - maybe it will help (see the 2x1.5" bar):




Also look at this tech article: http://datsun1200.com/modules/nsectio ... p?op=viewarticle&artid=45
Go down to the Hitachi Twin Sidedraft section. And that will point you to the GX tech article: http://datsun1200.com/modules/nsectio ... p?op=viewarticle&artid=33