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.