Do not weld it yourself unless you'd bet your life on your welding skills. Use only MIG or TIG welds and DOM or CrMoly materials. One bar breaking loose in an accident can kill you instantly. Never put someone in the back seat if you have a roll bar or cage. PERIOD. All the padding in the world won't save someone in the backseat from severe injuries when their soft tissues stretch and connect with tubing. Here are my recommendations if you are planning on running a roll bar or cage:
1. Get 5pt SFI-approved 3" harnesses (RCI, Simpson, Sparco, G-Force, and others make approved harnesses). The stock belts will not keep you in place during an accident. Even 5pt harnesses stretch during impact and you can still hit your head. The human spinal column and soft tissues can stretch a LOT in an accident. Never run a roll bar or cage with stock seatbelts. You need at least a 5pt harness. Do not use a Y-type shoulder harness as these have shown to discourage balanced distribution across the torso in a collision which can lead to severe neck and spinal injuries.
2. A fixed upright FIA-approved seat. Corbeau Forzas are very popular as are Recaros and Sparco seats. Look for the FIA-approved or Homologation certification. That means the seats are tested to withstand impact forces. OE seats flex TOO much and the folding seatback and sliding mounts can very easily fail in a collision.
3. Use FIA-approved roll bar padding.
http://www.rollbarpadding.com sells the best FIA-approved padding I know of. The "water pipe insulation" that most speed shops sell does absolutely jack **** to protect you in a crash. You need FIA-approved padding to absorb the majority of the energy transmitted if you do connect with a bar.
4. Use an adjustable seat brace that connects to the harness bar and make sure it is firmly attached to the seat back. This will discourage belt stretch due to seat flex in an accident and secure the seat to the roll bar or cage if the primary mounts fail. The seat mounts should use at least four Grade 8 Bolts and ideally should attache to the roll bar and frame.
Most certified welders/chassis builders charge at least $70 per point. If they have to strip the car, it can get expensive really fast (try upwards of $2000).