Buying My First Muscle Car 1973 Duster 340 Interior Pic

Hard to find any 50 year old that is "solid" unless someone before you has repaired all the rust, or it sat in a Texas garage for the last 50 years! Take a refrigerator magnet with you to get an idea of an amount of filler on any car that has EVER been repainted. Yes any straight car will have some filler, and any metal repairs will have required some filler (Bondo).
Realistically assess your skills, equipment, facility.. can you do any bodywork? for example. If not find a car that is done paint wise. 99% of A bodies will rust in trunk and inside floors, and most other models too!
You might find a good slant Duster that has all the body/paint work done, is a driver...and you can make it into a v8 car is that is your desire. Just figure what that will cost, and don't overpay for the project. Figure what it will worth done. After all said and done, any project might end up costing more than a done car!! Finding good projects and fair priced done cars ...Easier said than done!!! Ah .. the thrill of the chase!