sorry.
For Datsun Hitachi carburettors, you need fixed needles, these have two letters on them, such as SM, SH etc.
For SU carburettors, you have two types of needles, swing and fixed. The swing needles use a small spring to apply pressure to one side of the jet, this was done for emissions and low throttle response.
All Hitachi carburettors, standard on datsun, have a fixed needle, so it fits in direct to the based of the piston.
From what i have researched and found, L28 running Hitachi, try SM or SH needles.
Use this tool to play around with
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=15&cad=rja&uact=8&ved=0CDEQFjAEOAo&url=http%3A%2F%2Fhome.comcast.net%2F~rhodes%2FPDF%2FSU_Needles.xls&ei=duOfVNb9DpO78gWbv4GIBg&usg=AFQjCNEsDxM5cLMEf1XbD66CgA7Dm98joA&sig2=qP1KgmDjONW2aCgihXo7Kg