well even better if the rockers are brand new.
find out the valve clearance on the exhast valve then set one rocker up to the right clearance whilst on the head with feeler gauges like you would do normally. take your measurement off this tappet, then use a digital vernier to get the right size and make sure all the others are the same size to within about .1 of a mm or as close as you can. do the same with the inlet valves then but make sure you have the right clearance by refering to the haynes manual.