I used some aftermarket pieces but they are pretty generic pieces of steel. Just two pieces of 0.085" wall 1.5" x 3" x six foot long rectangular tubing and two pieces of 16 guage sheet 5" x 12" for doublers on the pieces of tin that anchor the aft bolts on the front seat. The doublers are 12" long and are roughly a "U" shape channel to conform to the seat support. The bottom is 1 3/4" and breaks 90 degrees on one leg. The other leg is more open and broke at 75 degrees. The rectangular tubing lays flat under the car so I cut the ends off at 45 degrees and boxed the ends to avoid their becoming crap catchers. Check the length under the car for fit first.
I started off by putting the car up as high as possible to clear my belly. Then I held the tubing in position with a jack and marked the final position with a sharp tool on the front and rear torque boxes as well as on the rearmost front seat support where it hangs down. I held the 16g doubler up centered on the final connector position and marked it for drilling. After drilling it I held it in place while scribing around it. Next I ground to bright steel around all the marks and scraped off any undercoat that might interfere with a tight fit.
Weld the doubler in first. I hammered on the edges to make full contact all around for easier welding. The car tin is really thin, tiny stitch welds work well.
On to fitting the connectors, the front end is about twelve inches from the rocker panel pinch weld, just at the rear edge of the front wheel well and as far inboard as I could get it while still maintaining clearance for my welding tip all around. The connector angles out toward the rear and ends about 3/4 inch outboard of the spring perch bolt. Make sure that there is room for the big washer on that seat nut on the doubler. I had to point load the steel with a jack to make it get full contact with the torque box. When I had four one inch welds I moved the jack and welded it out then moved to the rear and did the same thing. When that was done I forced the center of the connector up to the seat brace doubler and welded it too. The rear footwell gets pushed up a tiny bit in the process but it's not a problem. Now get back under there and spread some rust treatment, primer, and then the correct paint (GM chassis black) to protect the whole mess. It will be almost undetectable from outside, the only tipoff is a very tiny bit showing towards the rear if you get down low.
If you don't have a welder, a local muffler shop should be able to do this cheap if you supply the hot chocolate and donuts.
Now go and be amazed at the difference in how it drives.