here is what I know about the rocker arms. Keep in mind that the idea is to dribble/trickle oil out of the spit hole of each rocker to cool the springs and lube the shaft, ball, push rod and lifter seat. 55-57 rocker arms are different then the 59 and up rockers. The way you can tell the difference is to look inside the rocker. You should see a grove inside the rocker that intersects with the spit hole on a 55-57 style rocker. Not sure about 58, might be the same. In 59, Chevy changed the oiling system to create a sort of pressurized system. Oil feeds up through a passage in the block to the head in the 59 and up, through a larger orifice, into the rocker connector and into the rocker shart, instead of pulling oil pressure off the rear cam bearing and feeding it to the rockers through a steel line under the tappet cover on the 55-57 version. Also, instead of the open loop arrangement in the rocker connector on the 55-57, the loop is pinched off (of course every one pinched off the loop on a 55-57 just to try to get more oil to the rockers in the 55-57 when the motors got a little mileage on them and the flow dropped a little from wear and tear and clogged steel line) All of this was intended to create a little pressurized system in the 59 and up motors. so if you look into the bore of a 59 and up rocker arm, you will see that the grove does not directly intersect the spit hole but is offset. The now pressurized (so to speak) oil moves through the clearance between the rocker arm bore and rocker arm shaft and then gets to the spit hole. So if you use 55-57 rockers on a later 59 and up motor, you may end up with to much oil spitting out of the rockers, and not enough dribbling onto the springs, and rockers, balls and push rod. So if possible, you would want to use the correct style arm for the motor. As for the shafts, I am pretty sure they are the same size dimensionally for 55-63. That being said, I am not so sure you can just swap new shafts onto old rockers as I am sure there is some dimension between the shaft and the bore of the arm that must or should be kept to allow the system to work correctly. Just my understanding.
