[JensR]

Some things to consider:

Angle the whole assembly slighty (or a lot) upwards. This might offer a bit of space.

Use motors with non-concentric (eccentric) output shafts and arrange them so that they use the space better. (There's quite a bit of empty space between the motors now, so the motor housings could be rotated inwards with eccentric shafts.)
RB35 motors, replaced with 380/400 sized motors are a force to be reckoned with and have eccentric output shafts.
Como-Drill also makes some nice gear motors like that.