Running any resolution shouldn't matter on a CRT as the pixels are allways square, it just misses ones in between (hence if you run 640*480 you can actually see the black bits between the pixels). So using a 4:3 resolution shouldn't matter.
I run 1600*1200 + 1600*1200 + 1600*1200 and a switchable 800*450 from Svideo.