With the engine already being leaned on, I'd worry less about the SFC effecting the originality of the car, but focus more on the functionality they provide. That said a custom made set is probably the way to go. On my 70 Nova I made a set out of .120 wall 2x3 and sectioned the floor so from the side you can barely see them. Granted, section them through the floor is kind of a one way deal, but it is also very strong and kinda stealthy. The bolt in Comp Engineering ones are ugly. They hang down too far and I'm not sure they are all that effective. The Comp Eng weld in versions can get pretty involved on the install and might require other mods. For bolt in, look at the Global West offering. For pre made weld in go Detroit Speed if you otherwise go custom.
Global West (nice bolt in):
http://www.globalwest.net/1970-81_Ca...obal_West..htm
Comp Engineering (these look like pooh):
http://www.competitionengineering.co...?CatCode=10054
http://www.competitionengineering.co...?CatCode=11001
Detroit Spreed Engineering (nice stuff, but weld in)
http://www.detroitspeed.com/productp...b_con_prod.htm
I vote for weld in, either DSE or custom....
Chris