Distributor recurve

Doesn't surprise me that the engine likes more idle timing. Factory settings were conservative.
But you can have your cake & eat it too.....by connecting the VA to a manifold vacuum source [ MVA ].
You can add the extra timing that the engine likes with the VA. Say the static initial is 5*, you can add 15* with VA to give the engine the 20* it likes. The beauty of VA is that it is load compensated, & drops off under load to avoid pinging. You will probably have to fabricate a stop for the VA to limit the travel.

More on MVA, scroll down to post #6.

www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html