Either you have an odd truck or your mechanic is full of it.

My 03 4x4 has torsion bars. I thought they all did.

Not sure about your tire size. 35's are perfect with a 6in suspension lift. not sure about b/l.

Also I don't understand how a body lift could make that much difference in a crash. It is still connected to the frame just as much as it is from the factory.

The only difference is that there would be more torque on the bolts that hold the cab down to the frame. So what??? Those bolts aren't going to break. If you hit something that hard your truck is going to be all jacked up anyway. So what if they have to re-adjust the cab mounting bolts to line it back up again while they are fixing the rest of the truck???

It's not like it's going to come off the frame or anything......and don't tell me it could........

 

even IF the truck body were to slide off the frame, there are straps connecting the body to the frame to help prevent it from doing so.

 

All 97-03 4x4's have torsion bars so your mechanic is mistaken if he told you your truck doesn't. If your mechanic looked at the truck and told you this, he is a moron.
As for tire size, 33's or 35's look great with a body lift. The largest combo you can use is a 35x12.50 on a 10 inch wide wheel with 4.5 inches of bs, which is what I have.