I have tried to do it that way before and they did not crush down and remain installed. I know amk sells a tool for doing it but i thought someone might have a different way to install them. I think you have to use a couple washers and a grade 8 bolt but not sure. Just want to make sure before I try and install them because the local mustang house charges 10 bucks a shot for them.
Although others may have successfully installed riv nuts with just a bolt, the proper way to do it is with the correct tool. Using the right rivet tool will ensure the nuts are fully compressed and tight in the frame so the bolts will have a solid anchor to screw into. Without the tool, maybe the riv nuts will get properly seated, maybe they won't. If they don't you'll be right back where you are now. I bought a cheap rivet nut tool to install new riv-nuts in the door for the side view mirrors. It was tuff to find the right balance between compressing the nut and not breaking the mandrel but even though I broke 2 mandrels I know the nuts are in there. If you get a better quality tool than the 1 I bought from Harbor Freight that won't be an issue.
The last time I tried on my old coupe I just shoved the rivet nut into the hole and tightened the bolt into it. This did not work, now I am adding power steering to my big block car and I remember my past experience so I figure there had to be a better way to install these things. The shop I got the parts did not have the amk tool so i thought a home remedy someone had used in the past was my next method. but there seems to be some rivet nut tools available. i will check with autozone and oreillys to see if they have a quality tool available.
I own a riv-nut tool for the aluminum inserts. It's basically a pop rivet gun, with a threaded anvil. I don't see why you couldn't use a grade 8 stud, with a heavy plate washer, and a heavy hex nut, with some anti-seize on it in order to accomplish the task with the steel inserts. Or, If you haven't already done so, you could check your local tool rental for the availability of the correct tool? Let me know if it works.