If the last digit of that block casting number is stamped, not raised, you can take it to the bank that it has been altered. So if we assume "5" is wrong, then the original number must have been either "4" (a 400 with 4-bolt main webs, not necessarily a RA III), or a "6" (a 350!!). Someone who knew enough to alter that casting number would have also known enough to alter every other stamped number on the car, so all of them are now suspect and can't be trusted. All you can rely on are the raised casting numbers. Start with the 4 character date code on top near the distributor hole (which should also be raised) and make sure it ends in either "9" or "0" (and make sure it's raised and not stamped). Otherwise it can't be a model year 1970 block. Check the VIN tag and make sure the rivets look right - although it's possible to fake those too.
One shot you have is the frame VIN stamp, just because it's so hard to get to. It's usually on the driver's side frame rail, on the top between the frame and the body, on the rear part of the frame. If you're really lucky you might be able to clean it off and see it with a bright light and a mirror held at an angle. Some GM cars also had another VIN stamp on the firewall under the heater-AC blower motor, but not all - and this even varied by assembly plant. You have to pull the passenger side inner fender, or cut a hole in it, to see it - and it might not even be there.
Hate to say it, but for sure that block casting number has been altered so it's possible this isn't a Judge, or even a GTO.