Sounds familiar and might be the 65 Mustang GT in this video. It was located 25 years after being stolen and returned to the owner. The last owner had installed a new engine and other upgrades. The rightful owner paid the prior owner for the upgrades saying “I’d have put that much money into it over the past 25 years anyway”.
https://www.youtube.com/watch?v=6XRE3yw-PdM&ab_channel=InsuranceCrime
The one I’m thinking of was a plain Mustang, not a GT-350.................