Am I interpreting casting date correctly as March 63? If so, I have never seen a 261 as late as that one with factory milk bottle lifters. Those lifters were generally in early motors, think chilled iron sealed power at887 lifters for us with the early steel Cams. sealed power later on changed the material they were making those lifters out of and they could be used with cast iron cams as long as stock spring pressure. So possibly early lifters installed in a later block during a rebuild, but hey, I could be wrong. If they are, take a close look at your rockers. Using the solid side at887 lifters in a later block cuts off the oiling to the rockers.
If it is a later 261 block, you can use at874 hardenable iron lifters, has the proper side contour to feed the rockers. You do have to use the later push rods, 10 5/8 long, Napa 215-4011 or sealed power rp3020.
