Put it behind or on top of the gantry. Fitting it inside will be a pain, it will not get as much cooling, and maintenance on the motors will be impossible without significant disassembly.
The HRD (or other profile) rails are fine, but they require quite a bit more care during the frame construction. If everything isn't extremely flat/straight, they bind up or wear out much more quickly. That's why I suggested bearing rollers - they're more tolerant of misalignment.