Hi,
I think it would be nice if we can still morph the vehicle body after fixtures are placed. Recently I feel dumb because I just found out that maximum tyre profile is affected by how large the front and back fenders are. Often I didn’t morph them large enough at the first place then I found out I needed bigger tyres but can’t make those fenders wider.
This has already been requested before. It is said by the developers that it is very hard to do because it can cause a lot of glitches and bugs.
However, it might be possible in the future that they check if in the affected area there are placed fixtures and if so, warn you before being able to move it.
However, at this point not possible unfortunately!
We’ll indeed give it a try to make this possible with the biggest offender, the flares 
it might turn out to be not possible though
Thanks Wizzy and Killrob for your responses. I hope you can address that eventually, or at least give us some on graphical information on maximum tyre size possible at current size of fender.
Good luck!