What is Factory Five's recommended approach for interfacing a late model Mustang (e.g. '04 Mach 1) fuel filler neck with the Aston LeMans gas cap? The stock neck has a 1 3/8" OD at the tank, 1" OD in the middle, and 2 2/8" OD at the top. There are some posts at ffcars.com on this, but am looking for the factory's input (and it would be cool to see a FAQ on this). Thanks.