as terratonga said the l-series is heavier which means you loose the best aspect of a 1200 its handeling as it would become very heavy on the front, an l-series will go straight in a 120y but not 1200's, and 120ys dont have the same handeling stock as a 1200, id say go the a-14 and just chat to a good mechanic about what to do to the motor to get good hp, you really need to specify your price cause you can spend $1200 to freshen a motor or $5000 plus making it perfect, there becomes a point where small hp gains become very expensive due to fine machining requirements
anyway make sure youve got the right bits to start, i.e. you want an a-14, a GX head, twin sidedraught carbies if budget permits or at least a single downdraught webber (maybe others can suggest best size), its quite possible youll get twin side SU's with the GX head which are ok but not as good as webers or delortos.
then start looking at what you can afford to have done to the motor during the rebuild, i would definately look at a 5-speed and with that youd probably want a 4.875:1 diff centre out of an a-12 vanette just to improve acceleration cause lets face it its not worth having a car that goes 200kmh cause its just too flippin likely to loose your license with all the revenue raisers out.
we also must not forget the brakes and suspension with working the motors youll at least want to upgrade to stanza struts and brakes on the front and a 120y swaybar, which will not require a booster or change of master and will bolt straight in (see "how i upgraded suspension and brakes" thread posted a while ago by myself for install of these)
good luck finding a-14, test the car you get it out of for a 5-speed, if it has one dont i repeat dont take it, email me and tell me which wrecker it was at
you seem to want a straight answer on how to get good hp out of motor but one cannot be given, basically you have to specify your price and then a mechanic will decide what he can do for that much, for a mechanic its pretty obvious what to do as they will know what will provide best benefits with reliability, although theres many diffrent motors they all work on the same principles and basically can all be improved in the same ways.
that said youd be much better off taking it to someone who specifically works on datsuns or race motors and has experience with a-series, and you need to tell them exactly what you want the car to do and to drive like cause theres no point getting a huge cam which kicks in at 3000 like my motor if you want to drive in traffic and take off at the lights without spinning the wheels which im finding extremely difficult (my motors designed for one purpose "rally" and really the motor is not expected to drop below 4500prm until the end of the stage
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