One major problem I have seen whith chevys that most people overlook is the oil galley's! Most of the time when a chevy murders a lobe on a camshaft the galley's will get the material from that previous camshaft or the galley had been plugged with engine grime from use over the years and will wipe out the camshaft. 9 out of 10 times when a person does a cam swap on these engines they do clean the debris from the galleys and in return you have problems... Not saying this is your problem but something to keep in mind!