My factory service manual lists a clearance for the 273's bushed rockers, but literally just has a blank line in the column for the 318. I know these things are supposed to run loose on the shaft and none of the holes appear to be ovaled out, but is there any way to tell if they've worn beyond factory looseness? Inner diameter limit maybe? I've searched but I can't find anything.

There's not really too loose all the forces on them push them into the shafts and they never have a chance to get "loose" while installed. Unless they are obviously worn, they are probably fine.

If the holes in the rockers are not egg shaped or worn real bad on the side that makes contact with the rocker shaft, and shaft is not scored or worn to bad they will be fine. I have had some that seemed pretty loose and once they are installed the hydrolic lifters will pump up and and take the play away.