Low Vacuum from crate 360

More initial timing would be my first adjustment. Don't worry about total until you sort out the idle timing. The limit for idle timing will be when at operating temps, the engine won't start/drags starter or kicks back on the starter.

That set up should easily idle at 700-750RPM with minimal rpm drop when placed in gear.

It may have a bad vacuum leak somewhere as well. I'd isolate the intake tract from everything that is pulling vacuum and see if the idle issues clear up. If they do, reattach things one at a time until you find the offending system. Power brake boosters are a usual suspect in the leak area.