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Wow - that IS an amazing feat of dexterity. I'd still recommend pulling the head to replace a cam - and access the lifters from the top (perhaps with some long needlenose pliers). You'll also need to take out the radiator and grille so the cam can come out the front of the engine.
Before doing this, hwever, you can accurately diagnose a worn cam. Just pull off the valve cover and carefully measure how far each valve depresses as you crank the engine one full cycle (two times around). They should all be the same (unless you have some really particular performance cam with intake lifts less than the exhausts). The cams have a thin hardened layer and once that wears through the lobe pretty quickly loses its peak and the lift drops dramatically. From my experience it takes quite a while foor this effect to show up at idle, as the engine can breath just fine with the small lift. But at higher rpms it will start to run unevenly, sort of like a miss but not quite as extreme.
If you replace the cam also replace the lifters and gear, as well as the brass thrust washer that bolts to the block under the gear. Getting the timing gear back into the oil pump and in the correct position for the distributor is a black art.
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