I have a 454HO in the 59
Disconnect the vacuum advance and set to 16 as low an RPM as possible
I run 16 initial and at 2500 I am about 36-38-----with the vacuum advance hooked up I am close to 50* at 2500.
I have a manual trans so my idle is set at 750, altho many set their
BBC idles at 900-1000 with automatics
What kind of vacuum are you getting at idle-----me--only about 10