gday mate
this is my limited knowing of flywheels

the a14 and a15 flywheels are thicker then the a12
the a12 flywheel is lighter

if the bolt spacing is the same as a15 and a12 flywheel it should fit straight up ( im unsure about this )
but there is stories of guys using a14/15 engine with a12 flywheel due to its lighter weight so id say its the same

as for the gearbox if your using the a12 F.W and standard 4 speed then you would use all the standard gear, (clutch, pessure placte, thrust bearing etc)..
the starter motor mounts to the gearbox so using your a12 starter should also not cause any problems

before putting it all together make sure the a15 you get has a spigot bush,, if its out of an auto car it wouldnt have one

so if someone can answer the question.. is the flywheel bolt spacing for a12, a14 and a15 all the same?

hope you can make some sense of all this
cheers
Jason

edit: some of the questions were answered while i was writing my reply