I have owned my 74 Nova since 1978, and installed CE bolt-on sub frame connectors a few years ago. Before that, the front spring perches were never removed from the car. When I did remove them, 3 out of the 6 J-nuts broke. My J-nuts looked like the ones pictured with the hex nut (not the cone). Luckily, I was able to jam some flat blade screw drivers thru some of the holes that were in the frame to keep the nuts from turning while I removed the bolts (plus a ton of PB blaster).
After seeing were some of the original "hex" J-nuts broke, I knew that I did not want to use the J-nuts with the smooth cone shaped head. Some of my original "hex" J-nuts rusted thru the ears that hold the two halves of the J-nuts together. My fear was if these same ears would rust/weaken on the cone shaped J-nuts, and break during a future R&R of the front spring perches... there would be nothing to grab on to to keep the nut portion from turning.
When I want to purchase replacement J-nuts, all I could find were the cone shaped J-nuts.
So I went a different route and used new grade 8 bolts, nuts, and washers to reinstall the front spring perches. I also coated the bolts with anti-seize for same added insurance.
I've gone as far as buying some flea market 9/16" box end wrenches and bent them with heat to the length needed reaching through the hole on frame rail holding the nut.
When I did the sub frame connectors and ran into the J-nut problem, I remember reading a post on TC about "making your own wrenches to get the job done". Who knows..it could have been Everett's post I read a few years ago pointing me in this direction!
Since I was going the be using nuts and bolts to reinstall the front spring perches, I needed something to hold the nuts in place, and keep them from turning while I inserted and tightened the spring perch bolts. So I made some custom wrenches that could be inserted thru some of the holes in the frame to hold the nuts (with washers) in place while the bolts were installed. I had to use a different wrench for each one of the nut locations.
I used masking take to hold the nuts and washers to the wrenches.
Fast forward to this year. I installed some Caltracs, and removing an reinstalling the front spring perches was a breeze (I do have a special drawer in the tool box where I keep all of the "custom made" tools... since it could be a very long time before they are needed again).
I know this might be a little overkill, but I feel better knowing that I am using grade 8 hardware to hold the front spring perches in place.
Note: I believe the front spring perches used on the Nova hang about 1" lower in the spring perch pockets as compared to the Camaro's front spring perches. This did make it easier to get to a couple of the J-nuts thru the existing holes in the frame.