Anyone recommend some place that has accurate base coats for old Mopars (Plymouth B1 and B5 Blue to be specific) that will ship to my house?
PPG would probably have the most accurate formulas. Not saying PPG is the best to use but being they were around then (Ditzler) they probably have the closest in color match. I would start by going on the various Mopar forums and find out what others are using. Also you could give Roger Gibson a ring. (www.rogergibsonautorestoration.com/ ) He is one of the best out there with Mopar Restos and was very gracious to me once. He's a PPG guy as well.
Here is a good forum for B-Bodies https://www.forbbodiesonly.com/moparforum/