unless you can find a service manual for a car with the same motor i doubt it will be easy to get a advance curve graph for holdens to look at, and even if you did, it would be applicable for the equivelent of 4 or 5 star leaded petrol.... not the ethanol soup we have to use today.
as stated by blue and pete your best bet is more initial timing and limited total.. to take that into account.
few pitfalls that holden should be able to cater for.
the dizzy will probably have a standard spring and a long loop spring
standard spring lets advance come in fast up to a certain point when the long loop spring reaches the end of its loop and comes into action for the last 1-2000 rpm that it advances for i.e steep graph followed by a less steep graph.... once you have the total advance limited that long loop spring may become irrelevant, becasue the weights can't swing out far enough.
just add in a light spring with standrad loops instead.
if you plan to run vacuum advance and i think if it works, you should, (if the main focus is driving rather than racing definitly should given the cost of fuel) you may not need as much initial because the vacuum can will pull in a load of advance as soon as the throttle blades go past the ported vacuum port. especially with a standard cam in the motor.
more initial (which is a good thing) could potentially mean you now have too much vacuum advance which may or may not be hard to limit depending on which manufactuer made the vaccum advance can on your dizzy. some are adjustable via an allen key down the vacuum pipe and others are not. ask holden to check
when you get it back it will pay to go round and check cap, cap button, cap contacts, plugs, leads, and igntion power supply connections.
with a different curve you will be asking more or less of your igntion system at different rpm points
be a shame to be blameing the dizzy work when actually all it has highlighted is marginal spark power or grubby plugs or gapped too small.
just like the orginal advance curve the plug heat range specified in the manual for the car may no longer be appropriate for modern fuel blend
if using points igntion, a move to a "Hidden if necessary" electronic set up may make sense. (but beware of advertsing hyperbole or as i like to call it Bull.....)
loads of options some are just "eleiminate the points" upgrades, which are beneficial if its a dialy drive and you don't like setting or changeing the points, every 3 to 5 K miles, some actually provide more spark power...one focused on the latter is a much better investment.
buy once and never need to buy again....
i think in a poly 318 you have a nice semi hemi 1/3 of a sphear shaped combustion chamber traditionally they don't need as much total advance as a wedge or small heart shaped chamber, but static CR and your cam choice come into play here as well. if holden didn't ask maybe they already know.... but worth checking
in theory the 1957 1960 or 1966 service manuals here will cover a 318 A engine i.e a poly.
https://mymopar.com/service-manuals/
Dave