Yes, you can assume that the unfinished seat tube ID should correspond to the OD minus the wall thickness and then minus a fraction (usually .02mm) to allow for the sliding interference fit required. It's safe to assume that the manufacturer (or the shop that did final assembly) did some reaming of the ID to insure that the seatpost actually did fit and to remove any paint or other foreign matter inside the tube.
The best bet (if you don't find the confirmation from Sheldon or other internet published spec and can't wait for somebody here to give you 100% positive info): take it to an LBS that has a seatpost sizer gauge or use a few marked, known-sized posts in the age-old "go/no-go" test, but keep them greased and don't force anything that resists.
Cut to the chase: 90% certain that the correct size in a still-round seat tube opening will be either 26.7, 26.8 or 27.0 and between these 26.8 and 27.0 are the most common for Japanese seat tubing of this era...have to throw in the oddball 26.7 just cause it is a Miyata/Univega.