Honestly, after 275K miles, your engine is wearing out. I assume your Dad's Expy has had one owner and meticulously maintained with regular oil changes. These engines are excellent if you maintain them properly. But there is a limit. Metal fatigue is a real consequence of use. I owned a 1995 (or 1996) F150 that was the first roll-out of the 4.6 overhead cam V8 produced by Ford. It was a basic model, manual trans and no upgrades. This vehicle lasted through rough off-road use and two daughters learning to drive stick shift. After 2 slave cylinder changes, two clutches, replaced transmission and cracked manifolds I figured it was time to replace after a quarter million miles. The engine was still strong, but hard to justify continued maintenance of the vehicle. Like retiring your trusty steed after a successful life, it was difficult to do, but a necessity for family safety.
Whatever your choice you make, it will be the right choice. There is no wrong choice!!
