Car project: Chassis bolt issues, help needed! Video/ pics inside:

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Hi everyone, new to the forum. This is the first post I have made here. I have welded things before using a stick welder but not necessarily experienced. My background as to why I am on here, is because I like to work on cars. The car in this scenario is a 1990 300zx, if anyone might want to know. My issue I need help with is the following situation:Video of bolt, with head cut off so I could remove the bracket:Pics (second one of what the bolt looks like before it was cuthttp://s790.photobucket.com/albums/y...urrent=130.jpg:http://s790.photobucket.com/albums/y...urrent=125.jpgMy project car has a "subframe," which is a frame separate from the actual chassis, in which it cradle's/holds the rear differential and all rear suspension components. It connects to the unibody, by four mounting points which contain bushings. I removed the subframe to replace it with new bushings (these get old and cause handling issues). SO, to drop the subframe, you remove the four large nuts and lower it from the four large studs. HERE is where I ran into issues, there is a bracket also, which acts as additional support. You need to remove this as well. Each of these brackets mounts to each these bushing points, and the other end of the bracket into the chassis frame rails. I STRIPPED ONE OF THE BOLTS THAT SCREWS INTO THE CHASSIS. So I had to cut it off. Now I need to figure out how to get it out, and replace it. I think the other scenario could be: there is a nut on the other end inside the frame rail. This may have broken loose, so it won't unthread but just keeps spinning. How would one go about fixing this? Two scenarios I thought of:1: weld whats left of the bolt into place, then weld more thread onto it, effectively making it a stud. 2. Cut it all out and weld in another threaded plate of steel.Any suggestions welcome, I don't know what a welder would do in this situation. Thanks to anyone who responds.
Reply:usually the best way to access something like this is from above... it's pretty common for subframe bolts to do similar on older american sub frame and full frame cars. you will cut a window from the floor above to maintain the original strength of the pad that this gets bolted too.miller 330a bp TIGmiller dynasty 200DX TIGmillermatic 185 MIGthermal dynamics cutmaster 101 plasma cuttersnap-on YA5550 plasma cutterhypertherm powermax 30 plasma cutterbaileigh CS225 cold sawetc....
Reply:Looks like it may have had a resistance welded nut on the top.  I might try drilling a small hole through it and then use a really long bit to drill through the sheet metal above.  If the area is clear, then use a hole saw from above just big enough for a socket.My name's not Jim....
Reply:as stated about drill a hole through the center with a smaller bit and use a long bit of the same size to drill a hole through the floor pan, your going to have to acess the body nut that is typically welded to the body. Ive ran into this many times before. You should be able to access the nut from the top side and put a wrench on it. as far as the one that is snapped/cut, take a bit and drill trough the center. The find a nut and set it on the bolt (same size, next size up, i like using stick, but sometimes use the mig, crank your welder up and weld the nut to the remaining bolt, if you sink the heat in the hole you drilled it will help "shrink" the bolt.good luck bud
Reply:Yup, hate those welded to the body nuts!  Got to get to the top side like mentioned. Sometimes there is a hole close by to get a wrench in, like others mentioned sometimes you have to make the hole.Any case you need a hold by the nut to first get the old bolt out and second to get the new bolt/nut in.
Reply:OK, so from your suggestions above:I put a long wire up the adjacent hole which I was able to remove the bolt from. I can stick it up about 6-8 inches, so I am guessing that the frame rail isn't totally boxed, which is good. It looks like when I stick it up there it bottoms out on the sheet metal floor board of the interior. So, I should be able to get a hole saw on it and make some way to hold that nut. I will see what happens from there.Thanks for the help, and if anyone has something to add please do!
Reply:The broken bolt  most likely threads into a heavy nut that's a loose fit to the body, but  "caged" to keep the nut from turning.Anyway, if the broken bolt were heated cherry red and cooled, a few times, it'll probably turn out easily.Good LuckLast edited by denrep; 02-05-2012 at 09:17 PM.