Scott;
As a welder for many years..Please take the
oil pan off before trying to weld it..There are just to many things that can go very wrong trying to do it in car..A lot of good information has already been told to you; but here is one more things to think about..
The ground clamp to the welding machine. The welder needs to make sure that the clamp is as close to the work as he can get it..Do not let anyone clamp that clamp anywere other than on the
oil pan..
The welding machine will look for the shortest route back to ground...What that means it that if it is clamped say on the other side of your motor; it will more than likely arc across every bearing in you motor to get back to ground..Bye bye motor..
Good luck
Rick