If you're just after the performance, why not make your own? At one point I was planning to use a GTO dual snorkel and run hoses up into the grill, similar to the Olds W30 style. I read somewhere that the W30 style, with hoses going straight to the grill was the best design. Then, you don't have to get all that expensive fabrication for your hood and aircleaner. The trick though, is to find space around that big radiator. I think also, to get the optimum performance, you'll have to rejet the carb.

I've also heard that hood-mounted ramair can have negative air pressure at certain speeds- especially the early 70s Formula Firebirds.