Would it be a good idea to grind off the remaining stud and drill through the base, then tap the base to use a through-bolt ?
Can't get to the base of the stud, it's inside the cowl, I don't know how the factory welded them...........

I believe they were projection spot-welded on.
Would work if the remaining base was still firmly attached on firewall, and it's large enough to accommodate the tap hole.

This is a fairly common issue and I just repaired one on a 70 Challenger we are restoring for a customer in my shop. This is the correct way to repair this issue. As you can see the factory welded these studs to a plate and then spot welded the assembly into the cowling. I remove the assembly and then replace the studs and then reassemble.

How do you get the original studs off like you did?

Challenger Steve,
That is quite impressive !
I'm not going to be able to fix it like that unfortunately. Good to know though.

I just supported the back side with a socket and tapped the studs out with a hammer. The factory tack welds broke easily.

Worth a shot !! Ihave these brackets that go in the doors and they have three studs each and two are broken on each bracket.