The oil groove in the roller lifters is higher than the groove in flat tappet lifters, so I doubt that is your problem. I have heard that blocks that have run roller lifters can have trouble with the lifter bores when going back to flat tappet lifters...something about the roller lifters making the bores out-of-round or worn out. were the lifter bores checked out during the rebuild?
I did indeed pay special attention to the lifter bores with this block. Before finally cleaning the block, I honed all 16 lifter bores with a wheel cylinder hone to clean them up, just a couple of rotations to remove any deposits. Then I checked all 16 lifter bores with a new lifter and light oil (WD40). All bores felt just fine. Could I have looked over something here?