I tried to install descent 1 + 2, and I had DOSBox 0.73 installed n my system. I pointed the installer to that folder, but it said "DOSBox not found or too old", then proceeded to install DOSBox 0.72.
I found that you can work around this by temporarily replacing your 0.73 dosbox.exe with the 0.72, then replacing back when you are done. The 0.72 version IS recognized by the installer.

Ugh, I get the same issue with Gabriel Knight and a search for "dosbox not found or too old" on google only comes up with this very thread as a result. Where can I get just the exe file of 0.72, since the DOSbox website only has the latest from what I can tell, which is 0.74, and I wouldn't want to install the whole 0.72 version anyway. Just having the exe file kept in an archive for when it's necessary to install gog stuff.

I wish the installers for the DOSbox and SCUMMVM games allowed you to simply install the game files to circumvent such detection issues, I don't like having multiple versions of the same software installed for no reason and if games require such external software I get the latest version of that software to ensure the best results and then just want the actual game files from gog.

Update: well, I found the 0.72 installer and instead of using it to install the application I just extracted the files with 7zip, got the exe file, and now hold a version of the latest exe and this 0.72 version in a zip file each and simply extract the old version to overwrite the new with when I need to install a gog game and then re-extract the new version after that to actually play using the latest DOSBox. It works alright and is much simpler than it sounds so do that if you have problems as discussed in this thread.

I never tried out Descent 1 and II with DOSBox before. Descent: Rebirth really works with the game I've used and played for years without any issue. I think you should go to:
http://www.dxx-rebirth.com/

Yeah... Here's my old post from Descentbb.net: