I'm playing on a 4:3 CRT monitor and the game is letterboxed on the top and bottom of the screen- is this expected, or is there something I can do to make the game fill up the full 4:3 area without stretching?

Use the CRTs settings, to stretch the game to 4:3.
The games resolution is 360x240 instead of the usual 320x240. The devs expected you to calibrate your CRT to fill the whole screen, when running the game.