I am doing mine right now and for some reason I lost the intake valves only on the drivers side head.
I am replacing all the guides in the cover because that is what failed and caused the damage I do my own machine work. I have so far $800.00 in it and that's all the guides a complete engine gaskets set to reseal the engine I surfaced the head and did the valve guides replaced the 4 intake valves all other is in great shape. I am servicing the pass head to do. I feel heads on these should be done in a set.
but I did allot of shopping around as well I feel the trick is if your not sure what to do ask a pro a mess up after doing all this work if done wrong will most likely coast twice as much if not right or destroy the engine.
my input.

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1987 420 sel