You could get an AR23/VC1853 vacuum advance can. It will give you 15 degrees advance at closed throttle. That will allow a base timing of 16-18 and mechanical advance of 18 without going over 51 degrees advance at cruise.
But your numbers now look right to me. As long as you're not getting any knock, you're probably about where you want to be. And manifold vacuum is the right choice without pollution controls.
The only advantage of the AR23/VC1853 can for you would be that it is not adjustable and can't wander off. Since you now know what you need, the fixed can is a little more reliable in that way.
See this post for all the particulars on timing the GMC first-gen V-8s.
https://www.gmsquarebody.com/threads/ignition-timing-for-first-generation-gm-v-8-engines.14508/