I've got two old Bronco's that have the original Radiator overflow tanks made of plastic. Both are in need of replacement. During my Internet searching I ran across a couple Sites that might prove to be of some help for you guys building tanks:

Www.bungking.com
Www.coyote-gear.com

Just received the stuff for building a "Bomb" style overflow tank from Bung King. Great looking stuff. Will get some help and post the pics.