This is totally obvious. Junior blew the straight 6 engine in his pickup while street racing, so had to swap his friend's 460ci galaxie 500 (wagon) motor into it. Now he owed his friend a motor, so he found a 289 in his mom's Falcon and swapped that into the Galaxy. His Dad's Triumph was sitting around, not driven much, and had a 4 that fit in the Falcon, so Mom could get to her hairdresser apptmnts. One day, when Dad mentioned he was going to take the old girl on a spin...and Mom was at the Rick Springfield concert with the Falcon...he spent an all nighter swapping his other friend's borrowed pinto motor into the Triumph.
At this point, things were going well, until Dad noticed the old girl was down about 18 horsepower when drifting out of the turns on Pacific coast 1. A look under the bonnet, and he realized he had been had.
But don't worry. It all turned out great. Junior finished high school riding the bus and mowing the yard. Mom ran off with the drummer in his Ferrari and lives on a yacht. And Dad got the 460 pickup with the hairdresser as a seat cover. When Dad died this year - rest his soul - the Triumph passed down to Junior. Of course, the Pinto motor died a firey death many decades ago. Only fitting for a pinto part. Alas, after many months searching, he discovered the old Falcon - with the TR motor still aboard - is now a mooring in Del Rey marina for Junior's - Mother's - yacht. Mom says he can have the motor - which is totally indestructible - back, as soon as Junior finds another 20,000lb weight to anchor the yacht. A happy ending for every one...except the Pinto.
Like I said. Totally obvious!