If you can get the engine to tolerate the additional advance at idle it will help with cooling (as long as it isn't too far advanced of course). At cruise it typically helps with mileage and also cooling (although at cruise this is less of an issue).

Have you hooked up a vacuum gauge? Do that so you can see what your vacuum can is seeing in terms of vacuum and behavior. Having unstable vacuum is not going to do you any favors at idle if attached to manifold vacuum.

Just wondering if you might not have multiple things fighting against one another, or the build is just such that vacuum advance cannot be integrated.