Any tips on fitting the US Car Tool Sub Frame connectors?

Installed a set a month ago. They are nice, but a PIA on a 45 year old car with undercoating....and (IMO) overkill

Next time, I would simply make my own for the shipping costs alone. Rectangle box 11ga tubing on its side UNDER the floor with a little notching at the rear attaching area will more than do the job without even cutting thru the floor. I personally do not care for utilizing the floor as the 4th side by welding the thin gauge floor sheetmetal to the larger gauge connectors.

The California Flash has nothing more than 1-1/2" round tubing under the floor pan connecting the front and rear frame rail. Hundreds of 150 MPH runs (3 Wallys) with the wheels up....and NO twist what-so-ever.

Cheap to build, simple to fab, easy to install, and effective......Win, Win,Win,Win!
Simply my opinion, yours may vary.