Q. I am looking for Enfield 350 Classic, please suggest me if it is safe for Back Pain in comparison to other bikes

It depends on how you ride and duration of trip! Front seat is good it should not cause you problem even if you're riding bike for 100~150 KM in a stretch. Another point is, classic 350 let your backbone to be straight when you seat, it shouldn't not cause you Problem unlike other sports bike. Go for it, it is best in the segment. You can also try Bullet 350, that's also good, slightly cheap but handling is easy compared to Classic, and it has longer seat.

I own stock Classic 350 with no modification / enhancement whatsoever. Ridden almost everyday and clocked 20,000km in last 14 months. This is by far the most comfortable bike i have ever ridden! The spring seat is a delight!!!

The seating posture is upright, hence you don't have to bend, and upright posture is least pressure posture for back. I met with an accident just before I got the delivery of my Classic 350, but found it more comfortable in riding than the CB Trigger I previously had. Only thing is, Keep it healthy, dragging it is a real pain.