You need to grind the frame until it looks like the pic I already posted. If you weld a shock tower to that motor mount, the shock will be too far away from the frame and the tire will rub it when flexed... also, welding it to that steel, it will flex that plate back and forth until it snaps off.

Do it right.

Oh, and regarding the plasma cutter: They produced relatively little heat - ESPECIALLY compared to a cutting torch, so as long as the fuel line isn't in the path of the arc from the item being cut you should be okay. It's really the simplest tool in the world to use - way easier than a torch or welder. Hook the air up, slap the ground clamp to one of the leftover brackets you cut off, and just try cutting through it. Seriously, they ARE the best thing since sliced bread.