In the Building Editor, you're able to freely move parts along the X and Z axes by holding Shift and along the Y axis by holding Ctrl.
Through a tiny bit of modding, I've added this functionality to the vehicle editors!
There are some bugs related to symmetry but if you avoid moving parts too far from a base/cockpit part, it should be fine.
https://1drv.ms/u/s!AvuubHYsSlZUmXlLYmA ... i?e=VrNSv6