I figured this one out by taking a suggestion I found in a "how to" video about Dosbox. They said the aspect ratio correction only works if you use the "ddraw" or "OpenGL" render. I went with OpenGL and now the aspect ratio is perfect in fullscreen; black bars on the sides, just like it should be …

Osprey, Thanks for your reply, I appreciate it. After a few hours of banging my head on the desk, shortly after I posted this I came across the solution. I had long thought it was a problem with the drive mounts and so I checked the "drive" settings in the game's profile again. I noticed the CD …

This game is freeware (or at least, not available on GOG or any other paid site), before anyone asks. I have an ISO cd image of the game, which I have actually played once before, and it worked fully. So I know the problem is not a "bad copy." In any event, I installed d-fend reloaded as normal, and …