You can have a higher total gym bench for this plank exercise.
The lower the bench is, the more difficult this will be.
If you push your arms forward, make sure you can keep your core drawn in tightly as well as have your glutes isometrically contracted.
Your torso should remain motionless and your lower back should not hurt.
If your lower back hurts during the plank you most likely have muscle imbalances. You can learn about muscle balance and posture here:
Fitness Trainer Michael Behnken, MS, NASM-CPT-PES, CSCS