I used the US Cartool SFCs too. If you're considering them, take a look at the latest issue of Mopar Muscle (Jan 2017- go figure). There's an article in it about adding a Hemi to a B Body, which includes discussion about installing the US Cartool SFCs. The unique part of this installation is the use of 3M structural adhesive to bond the SFC and torque boxes to the floorpan. The adhesive route avoids the risk of burning through the relatively thinner floorpan metal. Mig welding is still used to tie the stiffeners to the heavier gauge structural components. I have no idea of it this is really a better route, but it's definitely a little different path.
This article showed up about 3 months too late for me as I went the fully welded and occasional burn-through route.
Good luck with your choice.