I would take Rallytwits advice and go directly for an E16 with e15et hardware and ecu
the E series is the ultimate lightweight incarnation of the A series, they weigh very
little in fact no more than a 3 bearing A10 datsun 1000 engine.

Im crazy about weight reduction like Rallytwit and had an interesting situation arise
when I was getting rid of my collection of engines on the last day in Melbourne.
I couldnt sell an A12gx motor which I thought was stock standard except later found
out it was a speedway engine and outlived the full spec race head that bent one of the
roller rocker shafts as it spun to 12K rpm yet finished the race behind a hellbent pilot.
The guy i gave it away to pulled it apart to reveal a meticulously knife edged crank,
custom I beam rods with slipper bike pistons of some kind and other mods like painted
on the inside with some oil lines to spray the cam and under the pistons.

after weighing many engines including A10, 12, 14 and a and e15s I reckon you cant go
wrong with the E series considering some drag racing pulsars were hitting close to 500hp
with their built E15ets.