Super Six HP gain?

Your rear end will live for quite a while if you don't beat it on a daily basis. Avoid doing burnouts when going around corners. Eventually you will need to upgrade. The 360 trans will work but you will need a neutral balanced torque converter as all 318's use internal balance. You will most likely need to upgrade to the V8 radiator but you could try the 6cyl unit and see how it does first. It's a pretty easy swap if you use the conversion mounts, a little more involved if you want to change the cross member.