Here is my RC18 with a 380 VXL Motor...
I made a 380 Motor mount that bolts on to the chassis.
There were two easy modifications I had to do to the chassis
First drill the holes to bolt in the 380 VXL RC18 Motor mount.
Then cut the upper chassis to fit the motor..
Here are some pics below.

RC18 380 Motor Top RView


RC18 380 Motor Top LView


RC18 380 Motor Rear View


RC18 380 Motor Side View


RC18 380 Motor Mount


RC18 380 Motor mount Bottom Bolts Close


RC18 380 Motor mount Bottom Bolts Far


Will be making my RC18 540 Motor mount the same way.