Could be the neutral safety switch is not properly installed or connected. Look for a wired switch plug terminal that may not be connected.
How do you know that it is your car?
Could be the relay or bad battery cable connection at the battery or relay.
It wont start
Your car wont start. lol
What have you welded ?
use jumpers
how does it stall if it wont start?
Only if you can un-rebuild the car. Once a car is rebuilt, it is rebuilt forever.
Once a car is rebuilt, it is rebuilt forever
catalytic converter?
car wont start
Don’t no