I'm going to jump in here and say that I ran heavy weight oil for the first 2-3 years after my motor was built (440 similar to yours with a melling high pressure pump, 6qt hemi pan, pickup drilled out and tapped for 1/2" pickup). I wasn't exactly up on the smaller details like what clearances he built the motor unfortunately. I switched to synthetic. I forget the weight, but it was not the same as the heavy weight oil I was using. I would get lifter noise under hard deceleration..ie floor it in first and left it slow itself down without shifting. I switched back to the heavy stuff and haven't had a problem since.

are you sure you have enough oil in it? maybe someone put an aftermarket dipstick in it and it's not the right length? I had one of those milidon ones once, the steel braided one, and I could never tell how much oil was in the darn thing.