Because they are stamped steel. You can use 273 rockers. I would NEVER use a rocker with a needle bearing on the shaft. I know its been done forever but its still wrong. There a many good rockers out there that dont have needle bearings on the shaft. Any of those will work.
Just contact Mike at B3 racing engines and get the rocker arm geometry sorted out. They ALL need to have it done. His website is very usefull. Check it out.