Hi Tom,
I looked into these guys when I started the build for my GT40 engine but went with the T&D parts instead.
http://www.tdmach.com/
BTW looked at the pricing and the FE rockers (406,410,427) were $ 310 each if I read the price list correctly, seemed very expensive. The T&D components are very similar in design to the Jessel rockers, they are both expensive.
Tony R.