Excessive camshaft wear is a common problem on the Rover V8 engine.

Regular oil changes with oil of the correct specification is essential for minimising this.

Sounds like "your" engine may be have been changed for a 3.9 unit. A John Eales engine may also have various upgrades, pity you can't find more about it.

I'm no expert but I assume it's possible to identify the original block size by the engine number.