How far in the hole 318 pistons

I believe Australian engines did not get hit with with too many emissions requirements until later in the decade and in general were built off the original LA 318 specifications.
built in a new facility with an engaged workforce.
it will be closer to the book than not.
if the car is as original as you say all you can assume is that nobody has been in there before you

1972 block will have been cast before all the crap in the press about "bullets on wheels" being sold to 16 year old kids as well.

as stated elsewhere the gasket thickness and hole size or assumptions regarding chamber size and shape might alter CR by a few 0.1


you see it on the hemi 6s as well

mid to late 70s CR goes down, piston disappears down the bore, core shift problems some thick wall some thin wall some with the filled in webbing for 904/727/small block bell housing dowels, regardless of if necessary .

short of estimating by doing a compression check not sure you can be any more precise

id buy the cam based on what i have researched can be achieved in my budget with the standard head and block
then make it so, with head and block machine work and the chosen pistons.
KB or sealed power do slightly taller pistons go for the flat tops, rather the the Mount Kilimanjaro shaped ones. Everyone i know who used them in a performance build had to cam it top stop it pinging. (maybe they just build crap motors :)

id guess its going to be in the machine shop dismantled anyway

roller cam might involve sleeved bores or some kind of oil control in the lifter gallery
custom pushrods?? may be some clearancing for them
adjustable rockers
top of valve guide work for seals and space to get more spring in
appropriate length valves
special oil pump/dizzy drive. to cope with cam material
fuel pump hardened lever or electric?

it was all the extra work( $$$$) that put me off doing roller cam on hemi6 presume its similar on 318 although way more aftermarket support so maybe not. it will probably cost you less than it would have cost me

apologies if i had made too many assumptions about what you are trying to do


Dave