I like helicoils, but I have had mixed results implementing them in tight spaces. If that doesn't approach doesn't work, here's an alternative method I used to deal with this issue on our Honda race car:
I cut a U-shape in the subframe near the busted captive nut with a chisel bit in an air hammer and then peeled back the flap. This gave me room to fish a new bolt in with needle nose vice grips and run a temporary bolt down tight with an air ratchet. Then I tack-welded the new captive nut to the subframe, pounded the flap back more-or-less flat, and tacked welded it shut. Remove the temporary bolt and replace with the correct fastener and torque.
Since your captive nuts are still in place, you could oversize drill them (less need for accuracy,) cut a window in the frame to access the backside of them, and put a new nut behind the captive nut. Use longer bolts, run them down tight, and then tack weld the new nut to the top of the captive nut.
I acknowledge that it sounds ghetto, YMMV, etc. Good luck!
Well Hey! Welcome. Cool wheels on that subie
"...just tack-weld it... ...cut a little flap..." Who do you think I am? Nik Blackhurst or Ben Modified?? 
My wheels are bone-stock, and Datsun's appear to come from a 2000/2001 2.5 RS. I am jonesing hard for the new Enkei RSF5 wheels...
And now, Learn me this: Steering wheel position
When I went to the base auto shop to replace my ball joints and steering rack (I work on an Air Force Base), I did the ball joints first. I drove up to the lift, made sure my wheels were straight, and then raised the car and set about working. I asked the main mechanic there a question about the ball joints, he walked over and turned the wheels for better access. I didn't think anything of it until I pulled the old rack off and realized it was not centered. I did my best at replicating the amount of turn on the rack, but appear to have failed miserably.
The rack (post-alignment) has the driver's side outer tie rod screwed almost all the way on to the inner tie rod; the passenger's side is much further out on its inner tie rod (~1/2" of threads visible on DS, 1-1.5" of threads on PS).
Turning the steering wheel, Left is 1.5 turns to lock, Right is 2 turns to lock. Is this a function of the rack, or of the steering wheel itself?
I'm re-replacing the steering rack on Friday. Is it possible that the steering wheel is pre-turned 360° to one side? And is that necessarily bad (airbag equipped)? How can I tell? Or, "Crash, you worry too damn much."?
TIA!
You should upgrade from the Base Mechanic to the LX, GT or Premium Mechanic.
