Manny has a video of one of the heads with loose valve guides and not a lot of miles on the engine. Some feel that a few got through production with very over sized valve guide bores. With the valves banging around on the seats causing side loads eventually stresses the valve to the point where the head pops off. They were a two piece valve welded together, one time of material to take the wear on the stem and another to take the heat in the head area. Dropped valves generally windows the block and that is not good. Back in the day, dropped valves in the old L78 396 engines was a problem but was mostly due to inadequate valve spring pressure for the high rpms that these engines were capable of. An engine with good valve guides probably isn't going to have problems unless you are racing it. In that case, you should be building a race engine.
For the difference in price, I too would be taking a look at a C7. At 460 hp, they are not going to be very far behind the 427 in the quarter mile.