Might want to take that "new" tube down to your local Ford parts guy and check the part numbers. Maybe it's from an Econoline van. Here's another post about Econoline van dipstick tubes for 351W conversions as a reference:
http://pantera.infopop.cc/eve/forums/a/tpc/f/5650045562/m/8190013815
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.