Mine is not a GT40, but I do have a SCCA legal cage for the Lola. One thing I noticed on the cage that you showed is that those foot pads where the bar mounts in is woefully inadequate, IMHO. I think they should be tied into pads with around 80-100 sq in of area and if possible, welded in place. Mine you can have a look at by clicking my build thread in my sig.
There is a good cage thread here:
Roll Cage Information for GT40 Please