SCCA allows weld in in all classes. But it can only be welded fore and aft. It is allowed to touch the floor pan. This applies to stock, street prepared and street mod classes. If you're playing in prepared, get a roll cage and gut the car...

IIRC there's been a recent rule change that allows a third (mid) point allowed to the subframe connector. I think this was about fox body 'stangs had an interferance issue with their seat so basically now the seat bolts to the SFC...

The quick way to install SFCs is to buy bolt ins and just weld them in. I went with autorust technicians 2x2 weld in kit and I had to slit the floors to allow them to pop through (at most 1/8", can't see underneath the carpet). Took me a one Saturday.

What gage metal are those connectors made from? They do look good

I made my own for the FrankenDuster. I bought 3x3 .120 wall tubing. I took cardboard and made templates of the floorpan, then traced the outlines on the tubing. They fit against the floor like the US CarTool units for a FRACTION of the cost. They are welded to the torsion bar crossmember, the floor and the rear rails. I drilled 3 holes in the bottom, 1/2" in size to make sure any water that may get in has a way out. They were epoxy primed then finished with seam sealer and undercoating. I wish I had pictures to show. They look like Plymouth built the car that way.

On a related note: Who would agree that the TALLER that the connectors are, the more rigidity they add? It seems in theory that 1x3 or 2 round tubes side by side are likely NOT as strong. This isn't meant to derail the OPs "Cheap and easy" requests, just an alternative question.....