[Finnster]

Before I decided I was going to do a 4ws setup, I was trying to do what you are doing with the XL chassis. I wanted to reuse my nitro revo motor mount and mount it on a skid plate which would be attached to the chassis braces. I few pita's as I remember:

The motor does sit that low, but as pic'd, its not elevated on a mount yet. Depending on the mount, it may have to sit higher, and maybe further out the TVPs (depends on gearing.)

The motor mount would sit close to the tranny. forget about getting to the motor's front screws to adj mesh. You need a mount that can slide sideways like a nitro mount. If you mount it on a plate and low to driveshaft, care has taken to get/make a narrow mount so it won't hit the axle. The chassis braces can kinda get in the way too (at least if I wanted to reuse my Revo mount using bottom mounted screws.)
Overall, it was hard to find a way to keep a 15 series inside the TVPs.

In the end, I decided to just go w/ a KD mount. For $20, it was far easier to do than making custom mounts and plates and BS. Also, it allowed me to tuck the esc and servo underneath and keep safe, altho at the expense of a high mounted motor.