Is it the same rebuilder that has done each of these?? Big block chevy's are known for cam problems, but four in a row is a red flag for sure. If you're told the block is in excellent shape, what is he basing this observation on?? Something is wrong for sure, and it sounds like the lifters aren't turning, or not spinning enough, and the cam is going. There should be enough spash to lubricate the cam during normal driving, so the oil passages are not an issue with this problem. Sometimes a soft cam lobe or lifter is a problem, but not this many times in a row, and it's different lobes/lifters each time. If it were spring pressure, it probably wouldnt take 1000 miles for it to happen...I have wiped out cam lobes with heavy spring pressures within 15 or 20 minutes. Everyone here has brought up good questions to ask, but all of them should be a no brainer for an engine rebuilder. I hope he is doing something for you money wise???? I think it's time to look for another mechanic.