Lightroom :: Down - Sampling During Export?

I use Lightroom to manage a digital archive of historic photos at our county library. The images are scanned as TIFs at 600 ppi at the original size; images <3.5 scanned at higher res. Those are the masters to which I add metadata. These images all have a long dimension of somewhere between 2500 and 3000+ ppi but they are not consistent in size.
 
Now I want to export a set of JPG files that are the same  physical size as the original, but a resolution of 300 ppi.
 
The way I did it before LR was to run a batch action in PS Image Size where I changed the resolution to 300, checked Resample Image and let it go.
 
I've tried a lot of different combinations in the LR export panel; changing the quality reduces the file size but it still shows the same number of pixels and the physical dimensions that it will default to is twice the size of the original. So I 'm interpreting that as it compressed the file more, but it didn't actually downsample and throw away pixels.
 
The only other way I'm seeing to do this is to select images of similar size and then telling it to Resize to Fit with a specific dimension at 300 dpi and "don't enlarge" checked. Or, continue using the Photoshop batch action.
 
[I also create a 1-inch JPG that we use for online display and an index book so LR will work fine for that. But our patrons often want a jpg copy of particular files and, unless they ask for the higher resolution version, we give them the JPGs at 300 dpi original size.]