$20K total is probably on the low side, assuming that you do this right and you do it yourself. Plan on $7K alone for metal and parts. If you aren't going to do the work yourself, you'd probably be better off just buying one that's done already. As far as the missing tags killing the car's value, it may move you down from $40K to $35K, assuming that the vin indicates that this is a real '71 383 Cuda and the original frame vin stampings are in place and matching. Galen Govier (among others) can restamp fender tags. Galen will only restamp a tag if you can document the options on the car via records, build sheet, etc. He's very picky.
If the car is what you want, then go for it (assuming that you'll be doing the work yourself). If not, put my name on the list of people interested in the car.
