I think there may be a difference in the type of clip we're talking about. The J-clip doesn't have equal lengths top and bottom, the U-clip does. The 1969 Firebird lower pan calls for U-clips like the ones pictured below.
I agree about the fiberglass lower pan, it's not very good, but I think that's all that was available for many, many years for our cars. I think they also made the upper filler panels in fiberglass too, until just a few years ago, when they made them in the correct steel.
