Looks like you got a real gem....First, I'm not sure how rare a 428 convertible is but some of the links below may lead you. The shop manual has a vin# decoder. I don't have it handy but I'll tell you what I know. You can probably figure it out using the links below. What I know is the 6Y means 1966, assembled in Wixom, MI (all were in 1966) then a 2 digit body code, then engine code (428 was Q), then a 6 digit "serial number". The data plate on the door tells even more. Post again if you need more info.
Surf around the following sites for more:
http://automotivemileposts.com/tbird...lections.shtml
http://www.tbirdregistry.com/main.htm
http://www.tbird.org/DataPlate/vdata.htm
Check e-bay for samples of what asking prices are.
Shemp