Ian,
Try these guys:
C&R Racing if you don't mind jumping the pond [img]/ubbthreads/images/graemlins/smirk.gif[/img]
They made a special order specific for GT40NA and had two remaining in stock.
I purchased, 2 weeks ago, one unit. They have one left @ $500 usd + freight. I have not installed it yet so I can't tell you how well it works. Alain of GT40NA (forum member) has experience with them and recommends them.
The thing I liked about it is the "cans" are on the top & bottom rather than the sides. This allows the radiator to have the inlet and outlet on the bottom and making it a 2-pass system. Typically our cars have the 'cans' on the side with the inlet and outlet at the bottom which is a single-pass radiator.
Contact:
Devon
Phone (317)-293-4100