Be aware some of the old motors required certain restrictions in their oil lines to maintain pressure. I had a similar problem with a 1953 302 GMC straight 6. After the rebuild it had various sized copper tubing and rubber hose here and there for oil line and the pressure seemed a little marginal. I had the tubing replace with factory spec size tubing and the oil pressure increased significantly. There have been a few threads on this in the past you could search for.