1. Tear down the block and look
2. If you seriously think you have some form of a power gain, you could always go for a dyno run and see if your numbers are out of the normal range for a stock 3.8 V6.
3. Try to see if the J-yard has a phone number or remembers the guys name and try to contact him about what components he had put into the motor. Most yards keep titles for vehicles due to legal issues, and most places won't allow a yard to scrap a vehicle without a title.. so odds are they have one for it if the car is still in the yard.
-1994 Ford Thunderbird LX 351W
-1997 Ford Thunderbird LX 4.6L