Drill will work, but you'll need to get things out of the way for access. Grinding the top flat will let you center punch & drill easier, as well as removing the surface that the welding attempts may have hardened.
It'll take time, but if you work up in sizes to avoid taking out the threads you'll eventually be able to pick out the last half moon shaped piece of bolt that's usually left because the drilling is seldom exactly centered.
If a tap isn't available, a spare bolt of the same size with slots cut lengthwise in the threads makes a good one use thread chaser to clean out the welded in nut.
To avoid rushing it & getting frustrated, remember what you said about this job being worth the whole car! Take breaks with your beverage of choice (grin) and it'll be done before you know it!
P.S. - hah! we're all typing at once again...
Last edited by sailor; 03-02-2013 at 07:26 PM.