I've read that in 1967, they made around 3600 Firebird 400 Convertibles. Anyone know a way to find out how many are still registered or on the road?

that would really be hard to say, there were 15,528 conv. made But How many of the 9,229 326 conv.made people dropping in 400's from different cars.

I dont remember seeing a break down for # of 400s where did you get this info?

I think it would be impossible from registrations to figure out how many 400s. You could determine 6 cyl vs v-8 from the Vin but not 400 or not.
Even if possible would need to check all states and all countries for complete list. This is what the first gen registry will be helpful for.

http://www.oakmediacreations.com/registry/