Ticking valvetrain noise from fresh engine

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?