I don't doubt you do a lot of the restoration your self, even with that, I don't see that car getting done anywhere near another 20k.
I have to agree with n9671x2 about the cost of restoration. Even if you do a lot of the labor yourself, I'm not sure you can expect to get a 71 cuda from this starting condition to restored condition for another $20K. I guess it really depends upon what kind of quality you're happy with in the end, but I know that I have more than twice that just in parts for my car. Of course, I went overboard, and I'll admit it. I replaced/refurbished virtually everything inside and out including rallye gauges, steering column, stainless trim, interior (from headliner to carpet), window glass, etc. etc. You name it... I most likely replaced it. Granted, I decided to go with a new 472 HEMI engine, tremec transmission, dana 60, an 4-wheel disc brakes which were a majority of the "parts" cost, but even the little things like fasteners, bolts, screws, plugs, bulbs, and emblems added up quickly.
If it helps, I have all my receipts documented in a spreadsheet and would be happy to get a copy of that to you for reference if it would help you to assess the project better.