Yes, there are a few die hards out there (four) who do care if the 318 in the car matches the VIN....but good luck finding one of those four when you come to selling the car.
That said, keep the 318 and sell me that now-useless-to-you 340.
X2 Pull the teen and sit it in the corner. Build that 340 and rock and roll. If/ when you sell the car you still have the original engine if the new buyer wants it.
