Does your block have screw in pipe plugs for the two short
oil galleys going down to the main lifter galley
oil holes?. If so, then a piece of solid material that's 0.0005 to 0.0010" larger than the short galley hole size can be pressed into the hole after removing the galley plugs. This material needs to be just long enough to sit below the main "top oiler"
oil galley so as not to restict oil to the rear main. Then just replace the pipe plugs back in the top of the galleys when through.
If you don't have screw in plugs and they're pressed in instead, this would be a tough one as I'm not an advocate of those pressed in plugs after having one come out due to oil pressure. Still....you could do the same blocking as mentioned above but use more pressed in plugs to seal the galley when through. You would probably want to "stake" the plugs to give a little more insurance that they don't come back out. If the motor was out and apart, then I'd say tap the block and put screw in plugs in it at that time.