You could install the US car tool braces without welding to the floor; if you take time to get a nice tight fit against the floor (they do require a little trimming) and then just weld the front and back, you would get most of the benefits of a SFC. There are numerous designs out there that only weld to those 2 areas and do not even touch the floor. Welding to the floor just looks better and does add strength but on a normal street car you wouldn't realize that amount of strength, plus I think it is arguable how much it would/does add.
70 Challenger RT 440-6, 73 Cuda (under const)
05 Durango, 99 Dakota, 01 Ram 4x4, 14 2500 4X4, 10 PCP Challenger RT, 01 Viper GTS ACR, 52 B3B w/330 Desoto Hemi, 70 Hemi RR, 61 Jeep FC170
9 x Challengers. AAR Cuda, 4 RR, 2 GTX, 4 Chargers, etc... (too many to list)