I received my adapter plate from GT40 Australia, but there
are quite a few companies, who make them. (351 to G50)
Kennedy Engineered Products, in California, makes about every type of adapter on the planet for a very reasonable price. I have used them in the past with great success. Good guys, and superb quality parts. They also offer built in house, racing clutch setups for G50's. Check them out. I doubt that you will be able to beat their prices elsewhere.